Sounds like a locked out NPC roommate to me. Or rather, someone who thinks they are and has convinced the game of that as well. Or maybe you have an NPC door with a private room marker lurking within and it's some tired Service or Role Sim trying to go "home" to hibernate after their shift? I would probably do a Save As, unlock the door/invite him in, and see what he gets up to as a guest. :)
If you have MasterController though, can you get some information about him through its Sim > Status reports such as does he belong to a household and what is his career or function in the game?
