I'm not sure what can be done about this kind of gross situation?

Then get a lock put on the door, hand out the key when necessary and check who uses it last. If this is an on going problem it could be some outsider, like a client or a disgruntled someone, who can move about undetected and knows that this room isn't being monitored. It might be understandable if it is confined to just the floors, but the fact that this mess is on the walls too, seems like a deliberate act of vandalism. That manager may be covering up for her sick husband because she doesn't have a way to take care of him so in order to keep him from being alone she brings him to work with her. If it is who you think it is, where it coincides with what has been happening, then check up on him every time he's in there and see to that he has someone to watch him. He really shouldn't be left alone so it would be an added safety precaution too.