It's always the squirrels...
I set up my prison building around the 20 day mark or so, bunch of beds in one room with a connecting room (No door) housing a table and chairs in it.
Whilst setting it up I failed to notice a squirrel hanging around, and he became locked in the prison building as it was constructed. My knee-jerk reaction was to make food of the little critter and be done with it, but I thought I'd leave him in there to see how long he lasts.
Well, I forgot about him and a day or two later a tribal prisoner takes up residence. This guy takes a ridiculous amount of time to score lucky on the recruit and it isn't until around day 60 that this guy finally gives in. When the notification pops up at long last that this guy had enough of pacing his cell for weeks on end, I give the prison building a proper check... To see the very same squirrel, still running around.
The little sod survived by eating part of the prisoner's meals as they were being delivered (Floor was done up with wood planks, so he wasn't munching grass/berries).
I gave in, and made an extension on the prison building with a wooden fence (Mod) so that the squirrel could survive off the grass/berries that grew there. The colony is about 160 days old now and he's still in there, being our mascot and keeping the prisoners company.
I imagine when the colony finally burns to the ground, he'll still be there.
Wandering.
Eating.