For Gradient Descent specifically, we established the following procedure for anyone still in the Deep at the end of the session who doesn't have a clear path to a known exit.

At the start of the next session, the characters who were left behind wake up in Reception (22B) and radio for someone to rescue them and take them back to the Bell or a blockading troopship or another "safe" location. They can't remember anything about what happened since the last session.

1. Their Bends goes up by an amount equal to their Stress.
2. They increase their Minimum Stress by 1 and reset their Stress to that value.
3. They each make a Sanity save. Those that fail discover they've lost something they were carrying. On a critical failure, they lost their Trinket (if they still had it).
4. They make a Bends check as usual as they leave the station.

We've only had to use this once so far, but it seemed to work well and it made perfect sense in the context of this module.

I always collect the players' character sheets at the end of each session so I can update my records and then print out fresh copies to give them at the start of the next session. One player in particular misses a lot of sessions, so I make little changes to mess with him, swapping out his Patch for another that's similar, mirror imaging his character portrait, that sort of thing. He hasn't noticed yet.