Not as far as he can see.

Has the door been opened recently? Has anyone passed through it? Again, Toff has +31 on Perception checks.

Yes, and recently. A party of four passed through it twice, once in either direction. One was wearing robes.

Toff will retreat back to a different room while his elementals open the door, scout around, and come back to report on what they find there.

The elementals don't come back.

Did his +31 Perception hear any disturbing noises?

None.

Toff casts negate aroma and silence off of a low-level utility staff that he gives a wizard minion to recharge during downtime (the silence he casts on a rock that he could drop at a moment's notice) and then flies, invisibly, towards the door he lost his elementals through.

Is it still open?

It's still open. There's nothing in particular to distinguish the dungeon beyond the door from the dungeon he's already explored, stylistically - it looks Thassilonian - but his Perception modifier is high enough to notice that the floor is tiled in pressure plates, there are holes in the wall for poisoned darts, and vents near the floor for poisonous gas. Indents on the floor and walls imply to him that the ceiling can be dropped. A hidden hinge in the floor implies that the floor can be dropped to make room for the ceiling.

There are probably more traps which he doesn't have an angle to notice, even with his bonus to Perception. And there may be traps that he didn't notice; he only barely noticed some of these. 

...That seems excessive. But, those four guys went in there and came back out, and they almost certainly didn't see the traps, so maybe they're disabled or defunct? He'll approach near enough to look through the open door with arcane sight.

The walls, floor, and ceiling of the room behind the secret door all seem to be part of one magic item, with a CL of 28.

...

Toff returns to the entrance of the Player Character Cave and casts dimension door.

He's going to bug Zenderholm about getting Salgar raised from the dead, Toff wants more minions. And he's going to ask around and see if he can find any Korvosans who knew those guys (you know the four) closely before they came walking out of that dungeon.

Zenderholm would love to raise Salgar from the dead, but she needs 15 minutes to prepare the spell and 10 to cast it, and she's been kept busy.

People who knew the four guys before they came out of the dungeon aren't super hard to find - Toff himself knew Lyvina, so there's one right there - , but Toff is unable to find anyone who knew them closely.

(In fairness, though, it really wouldn't be that odd if everyone who knew them closely is dead!)