Sim prisoners - tips needed to prevent escape

I decided that the best way to keep my vampire happily and easily fed is to kidnap some sims and keep them prisoner in the basement. Don't worry, it's a nice basement.

https://i.gyazo.com/e3fe447aeaccd02ab6f2fbd0051e931b.jpg

The only door to the upstairs is locked to all but household members. This works most of the time. It takes a bit of effort to get the prisoners down there, but once they are there, they stay put.

Except, it seems, when there is a travel trip off the lot, or when the game is newly loaded. In these cases, when my lot is loaded again the prisoners are either completely gone (in the instance of travel) or leaving (in the instance of loading).

As you can see above, one of the prisoners is going up the stairs - she never should have been able to use that door! It is for household members only. Any ideas on how to keep the prisoners down there? It's a pain to get new ones in there, although I have some ideas for making it easier.

I just trapped them upstairs so they would have to come back down when I noticed them escaping. I removed the door at the bottom completely and was going to put it back after they came downstairs. Unfortunately, while one prisoner walked through the opening in the wall where the door was, the other walked straight through the wall! Do doors/walls even matter, or can the prisoners occasionally just walk right through them?

https://i.gyazo.com/e817c5f1443dd6885293bbbf37c7045a.png

Thanks for any tips! My vampire appreciates them.
Sineea