The next few days, leading up to Tony's victory tour's stop in Four, see her ferretting out her remaining stash and distributing it between her parents and Lynnis's family until it's all gone. They're assuming a ship lost a lot of merchandise at once. Bad weather, one assumes.
She stops replacing her shells by the doors. But she does show up to work, until the day before when she has to leave. Then, she fakes sick enough to stay home, spends the day sleeping, and - at night - is rested enough to execute her plan.
Dear Mom and Dad.
I'm going for Atlantis. It's okay! I have a plan! I'll come back and get you when I can and then we can all live there. Please tell Mr. Carrasen I'm sorry about the canoe. I'll bring him a new one after I get there! I love you! It's going to be great!
- Shell Bell
...the idea, after all, is to instantly convince them that she is dead. That she is too foolish and unprepared to have any chance. That there is no point in worrying about her any further, and they should move directly to grieving - certainly not wasting time looking for her or reporting her as a missing person. The fact that she's not swiping any of the cans and only one water container - is not even bringing a fishing rod - is another clue, of course.
Shell Bell wears her nice dress - it'll attract less attention in the town than work clothes, on a Victory Tour day where everyone's supposed to celebrate, and the town in question is big enough that her mere unfamiliarity won't catch her out immediately. She packs one more practical change of clothes, sticks her stick in her hair, and puts one salty roll in her pocket to eat at around midnight and a few strips of dried salmon to nibble on as necessary through the walk. She's also wearing the less remarkable-looking of the two protective amulets she traded for during her stay in Milliways. It looks like a lump of white glass - it could even pass for sea glass. The other stuff she obtained is waiting for her in her room in the bar.
She melts the lock on the canoe shed and buries it deep in the sand. She drags out a canoe. Briefly, she considers actually canoeing to the next town up, but while she knows how to row like any District Four resident does, she's worried about being noticed on her way in, and it'll be easier to avoid that if she's not in the place where all the actual industry goes on, namely the shore. She pushes it out to sea regretfully. Saltwater spatters her dress, but it dries as she goes.
She talks to her recorder in the dark silence.
And just before dawn peeps over the ocean, she's picked her way through the sleeping town to the train station and she's pretending to be a premature, eager celebrator.
Goodness, Bell is full of questions.
"I am extremely good at cooking. Although," she says with a slight smile, "I am rarely called upon to demonstrate as much."
"I wouldn't think so." Bell waits a beat, opens her mouth, then closes it. Sherlock is sufficiently observant that she probably didn't just forget the rest of the questions. Bell does not want to antagonize her hostess.
"Tony has no set theme for his public creations; he makes whatever comes to mind. I find the subject somewhat tedious."
"But presumably you live with Tony and could be interested in stuff he makes without interacting with publicity."
"And yet, I can immediately tell what he is making for himself from what he is making for everyone else."
Bell tentatively determines that Sherlock might not want to talk to her right now. She snuggles down in her chair, turns the volume way down on her recorder and holds it up to her ear, and has it start reviewing conversations she had in Milliways at 3x playback speed.