Of course, any such system would have cracks.  The Chelish government squeezes as hard as it can, but in some ways there effort make some cracks wider.

The obvious temptation to the Chelish government, once such a surveillance system was in place, is to use it not only for major crimes like smuggling persons, but also for minor crimes, like contraband goods.  This creates lucrative and lesser opportunities for rule bending.

A posting on a ship would be even more ill-regarded than a small village posting, except for the fact that a savvy cleric can bring in quite a bit of money knowing just when to look the other way, so the clerics most eager to apply for and stay on such ships are often some of the most corruptible.

The Chelish government did manage to impose mandatory mind reading on the captains (although even this much was a stretch), but there are too many first mates and men closely trusted by their captain to mind read them all.

Actually verifying the lists of names and family members is too much of an effort for anything but the most sparse spot checks.

And considering how hard the government comes down for even some novice seaman wandering off in a foreign port, everyone knew it was best if they "actually" died to a monster encounter or storm.

So all of this is to preface how we have come to the current situation.  A sailor, by the name of Marit, who was unofficially, apparently, actually a laundry wizard.  A detect magic would suggest he lacked he even the spare spells for an occasional endure elements, but somehow, occassionally damaged items brought to his room that were a bit too big for a mending left repaired.  When the first mate needed to go get a spare scroll to get a casting of tongues (for some questionably legal chance trade with Merfolk), that first mate somehow found himself going to the room of this sailor instead of going for the lockbox with the scrolls.

If you asked the captain, even under detect thoughts, he would be quite sure this sailor wasn't higher than first circle, well make whole is second circle, not higher than second circle.  (If you asked a few months later he would be much surer that this sailor wasn't higher than 1st circle, any apparent make whole's somehow forgotten).

If you asked the ship's cleric or the ship's official wizard, they would be much more certain this sailor was just a washout laundry wizard, basically doing the job of an ordinary sailor and not really a wizard, so not really a violation of the rules, and someone of their station shouldn't have to labor over dirty clothes or damaged trinkets.  They would have a few dollars to help you see it this way, if there was any doubt.

If you asked the ship's (less trusted) crew, they might repeat rumors about a noble's idiot sorcerer son who needed some experience in an out of the way location.  They weren't to mess with him, or comment too hard on how his shifts were shorter.

If you ask the ship's first mate... well somehow the official first mate and the man the captain actually trusted (and paid the best, taking into account illicit money) were not the same person.

So if you were somehow able to sneak around and spy on the crew of the Rose of Erebus, you might notice a laundry wizard with a spell scaffold of a third circle wizard, tweaking and tinkering as he tried to improve his time taking it up and down.

A young ratling, a little less than two years of age, who has been called many things but has no name as such, is also a traveler aboard this ship, unknown to its crew. Living in Ostenso, the city of his birth, was a frustrating experience for a variety of reasons. Having eavesdropped on enough conversations to learn that (1) Ostenso is part of a country called 'Cheliax', (2) there are other places that are not part of Cheliax, and (3) if the rumors are to be believed, some of them seem like much better places to live, he swiftly made plans to leave. Life is short, especially for a ratling, and some chances are worth taking.

When you're tiny, and also have invisibility and dimension door as spell-like abilities, sneaking aboard a ship is not particularly difficult.

Like all ratlings, he has constant detect magic. Which is why, when he's sneaking around the ship, scoping it out for threats and opportunities (i.e. things to steal), that spell scaffold catches his eye. He's only seen the like a few times before, and always attached to important people. So why is this guy (seemingly) a lowly sailor?

He'll keep an eye on this guy from a distance, and follow him around for a while (so long as that doesn't put the ratling at risk of exposing himself). What's he doing?

Right now, he is finishing up poking at his scaffold and leaving his room for his shift.  Apparently working some shifts as an ordinary sailor helps with the layers of rumors and bullshit he is hiding behind (as confirmed by both explicitly discussed planning and occasional detect thoughts on the inner circle of sailors in on the plans), and compared to staying in his cabin all day every day he actually prefers some work.

He works his shift just like an ordinary (albeit very inexperienced) sailor.

Once back in his cabin (one of the few ways he stands out is a very small but private room) he recites some odd poetry to himself.  The language is bizarre, almost as if he is making it up on the spot, but the structure is too consistent to be arbitrary.

Does tongues reveal the meaning of this poetry?

It's somewhat hard to understand because every few words there is a made up nonsense word, made up simply to rhyme correctly or have the right cadence.  Concentrating through this difficulty, it seems like an (initially) silly poem about a group of birds.  A lot of repetitious early sentences describe silly incidents and adventures the birds have together. Eventually some serpents that hated the bird's singing hunted them down while they slept, until only one bird is left, afraid to sing.  After many lonely wanderings, it flees across the sea and the poem ends.

There is an odd feature to this langauge... some of the nonsense words actually have clear meanings the second time around, the tongues revealing their meaning as if it was an ordinary word.  Some of the nonsense words don't translate cleanly to the tongues even after being repeated, but if the ratling sounds these words out in pieces it will turn out that these words are each a mashup of words the tongues can translate.

This is bizarre and incomprehensible behavior. Not like, dramatically more puzzling than several other things the ratling has observed humans do and doesn't have good explanations for, but he's never seen another human do this specific thing. He has heard that insane people will speak nonsense sometimes, but none of those overheard conversations implied anything like this level of effort and purposefulness. Is it some obscure form of magic? Is this man not actually a human, but merely pretending to be one?

If the ratling regularly spies on the human, it can see this behavior regularly, the human tends to do some form of incomprehensible recitation at least once a day!

Some general trends:

• Sometimes the human repeats previous recitations, but with some of the words changed to new words.
• New words usually need to be repeated at least once for tongues to work, but some words remain meaningless and sometimes new words are already translatable by tongues
• The human sometimes uses a casting of tongues or of a similar but longer duration spell (not one the ratling has ever seen before) to help himself, sometimes he does it without magical help.
• Lots of the words are changed when he uses tongues, but the ratlings tongues can translate them on their first usage.

He does regularly spy on this human, who is the most interesting thing aboard the ship, although he can't get too close, or follow him around everywhere. He only has so many minutes a day of invisibility, and it's not always possible to remain undetected without that.

If there's an opportunity to take a look at the human's spellbook when he's not around (and it doesn't have an alarm or something on it) the ratling will do so.

He typically hides his spellbook on the ethereal plane with secluded grimoire whenever leaving his room and whenever he goes to sleep.  He does have to leave his room unexpectedly one day, using up an extra secluded grimoire in the process... and that night he sleeps with his spellbook under his pillow.

Well, he's not going to try to move the guy's head out of the way or anything. It's getting pretty clear, by this point in the voyage, that whatever mysteries this man holds, he's careful enough not to give anything away.

The first mate has a job for him.  They had previously agreed any spell beyond first circle and outside of a few standard spell' Fernando could cast from the safety of his room would be paid for at half the price of a scroll, which is an absurdly good rate for Fernando, especially considering how he needs to make back the money he spent for his illicit passage (to save towards purchasing the materials to replace his bonded object).

According to the first mate, an unusually large rat was seen.  The ship's cleric has done the regular channels to try to kill vermin and they have only managed to kill regular rats, so something unusual is going on.  Fernando's half-joking guess was undead rats... but it seems the first mate quite seriously suspects some unusual magical creature that might be a danger to the crew.

Either way, the first mate agreed on the value of two second circle spells and one third circle spell just for Fernando to give this problem his best effort.  So Fernando, making use of his specialty, has a summoned spider swarm sweeping the ship one night (when both the ship's cleric, ship's wizard, and captain are all safely asleep).  Summon swarm has the unusual advantage of lasting as long as you can concentrate, so as long as he is systematic and careful he can sweep the entire ship with his swarm.  Hopefully the dire rat or whatever it is isn't immune to spiders.  If it is, Fernando will summon something with his summon monster III.

His swarm does find a few mundane rats, scurrying around looking for food, although it seems like the cleric got most of them.

The ratling is sleeping in a hidden nook between several crates in the hold.

He doesn't actually know what his swarm is finding until he looks himself.  And the only indication he has that the swarm has found a living creature is that it slips from his control and attack automatically.  Really, it's a testament to his skill as a conjurer and tweaks he made to the spell that he can direct the swarm at all, summoned swarms typically just attack the nearest creature without allowing any guidance from the caster. 

The first few rats the swarm finds and kills get his nerves up, as he struggles to direct his swarm onwards after they finish the kill.  But he thinks he has found his rhythm by the time the swarm surges into some small nook.  Perhaps this is a nest of rats the cleric missed?

The same tweaks he made to summon swarm to make it more controllable also means they have a slight delay in going fully on the attack.  But it's not like a rat or even group of rats will be able to do much with a moment of forewarning before the spier swarm fully closes in, is it?  He's on guard anyway, just in case the first mate's instincts were correct and something unusual is going on.

The ratling is woken by the spiders and immediately dimension doors to his secondary nest on a different deck. He had planned for something like this to happen, although the massive swarm of clearly magical spiders suggests a more potent threat than he anticipated.

All Fernando finds in the small nook his spiders were so exicted about a moment ago is a small collection of cured meats.

Almost as soon as his spiders start to attack they stop.  The rat must have died unusually quickly?  But no, he’s not seeing any rat corpses.  Did he imagine it?

He was sure he felt his swarm start to attack!  He carefully uncovers the coin he precast a light on.  Stockpiled food…

Whatever it was, it was intelligent!  And probably a spellcaster!

He stops concentrating on his swarm and casts detect magic, expecting to find some combination of illusion and abjuration to hide from the swarm and turn invisible…

But it’s conjuration!  His thoughts grind to halt for a moment as he tries to think of a summons that would enable one to vanish, before his mind jumps to the obvious thought that it was a teleportation!  Which means this “rat” is a more powerful spellcaster than him!

As he works through implications while considering his next move, one thought stands out.

A tiny magical creature with the planning to stockpile food and ability to cast a fourth circle spell might know a rare language.  Considering Cheliax… if it’s the last of its kind… killing it could mean extinguishing a rare language which would make it Anathema.  He’s really, really banking on his god making a strong case for him so he absolutely cannot afford the risk to killing that magical creature!

He thinks the first mate is actually pretty reasonable, he just needs to figure out how to spin this.

Once the panic of being awoken to a swarm of spiders subsides, the ratling goes invisible and has a look around. Who was responsible for this? There are only a few casters on the ship and only one of them is anywhere near the hold he was sleeping in.

Of course it's that guy. The mysterious caster with unknown magical powers is now looking for him, or at least has some idea of what's going on. And he's probably lost his stockpile of food. Hopefully the ship will make landfall soon.

They aren't that far from Corentyn, and then from there, they should turn south and then east making stops along the way, possibly in Rahadoum, likely in Thuvia and Osirion, and definitely in Absalom.  But a significant enough shift in weather, prices or major world events might see their entire planned route change to instead take them to Korvosa.

Fernando would be most happy with Absalom, but his deal with the first mate allows for any place outside of Cheliax or Nidal.

The next morning he approaches the first mate with his report.

"I got several rats, and then found a nook with food stashed away and something teleported out, I think a dimension door, before I could kill it or confirm what it was."

 The first mate thinks out loud.  "A gnome sorcerer?"

"I think the nook is too small... so some type of magical beast... which brings me to my alternative idea"

The first mate tenses for a moment, then smooths his expression, 'Marit' has kept to their deal and kept his head down, so hopefully his idea isn't too reckless. "I'm listening."

"This creature would likely be pretty troublesome to hunt down and kill.  And... I am, at the moment, without a familiar.  Magical creatures often make good familiars, so I could take it as a familiar and keep it out of the way.  If it doesn't find that agreeable, well, nothing stopping us from hunting it down instead.  Although... seeing as how dimension door is fourth circle I would need help putting it down." 

Fernando knows the general details of the ritual to bond with a familiar, and he has the materials from his efforts at collecting the resources to bond with a new object, so it should be doable... at least assuming bonding with a new familiar is less costly then replacing an bonded object... it's the best plan he has.

The first mate thinks it over.  "Don't give away that we plan to kill it if it doesn't agree, obviously.  But fine.  And if it can't pass for some more normal animal, keep it out of sight so it's presence doesn't contradict the laundry wizard story or idiot noble heir sorcerer story.  And I'll think I can arrange to obtain the materials you need for the familiar ritual, you don't have enough on you, do you?  I can take the payment from what I owe you for your last few spells, and the next several ones you cast... we'll call it worth 10 circles of spells."

Fernando winces, but nods in acceptance.  The biggest expenditure he was saving for was replacing his bonded object (he loses nearly a third of the spells he tries to cast without it), so just as well if the money he would have earned went towards a familiar.

So, that evening, with the cleric and ship's wizard confirmed to both be out of the way, Fernando finds himself searching out nook after nook of the ship, repeating himself in multiple languages.  His first guess is a Fey, and he would take it as a sign of Divine favor if it was a Protean, but he has is resigned to the possibility of an Imp, so at each nook he repeats himself in broken Sylvan, fluent (if slowly spoken) Protean, a school child's Infernal, and fluent common Chelish Taldane.  He repeated (with some variations for lack of or overabundance of vocabulary):

"I would like to talk.  The ship's captain wants you dead, but I have an alternative deal.  Please respond." 

Eventually, in an isolated part of one of the ship's lower decks, with no one else around, he hears a quiet voice from behind him (in taldane).

"I'm listening. But know that I know secrets of yours, so don't think you can take advantage of me."