My 92-year-old father lives in an assisted living facility in the memory care section. Residents are going into his room.

A simple solution to this would be all rooms lock automatically and a resident wears a wrist band that allows them to enter only their room. Plus staff would have access to all rooms with a key fob or whatever. The problem is these places cost so much money but the adminstrators don't want to invest in technology and solutions that can be of a benefit to residents (if it's not making them money). Dementia or not patients should not be allowed to wander into other peoples rooms like this. With the technology we have today there is no excuse for this.

Of course other residents are going in your fathers room. That happens in pretty much all memory care facilities, as folks wander and don't remember where their rooms are or where they are. It's sad to witness.
Unless you have a camera placed in his room(which not all facilities allow)you will never know exactly who is responsible for what, as other than the staff, everyone else's brains are broken and can't be taken at their word.

Please understand that your Father may not be recollecting events accurately. Maybe. Ask if you can install a camera in his room to see what's going on and who is really liable. He's in MC, so you already know his memory is not good.

speak to the administration. See their point of view of what is going on

if you think there is damage/ theft going on - will need to read your contract to see what it says about that, as well as - ask administration.