Apocalypse Fic Question: Is there a way of keeping those Sims INSIDE the Bunker?

I don't know how it is...but every time, I roll-call for lights-out at night, invariably one or more of my sims "Houdini" their way out of the bunker. Is there ANYWAY to keep the little weasels in the bunker for good? Or do you have to do a perimeter inspection every single cotton-pickin' time to make sure that none of them get out?

Edit: The door is LOCKED...and shut. The only way they're doing this is to weasel their way through the foundation which is the Sims equivalent of four feet thick.