Ghost, or someone possibly living inside your walls?
Or someone who has access to some secret door to your place that you don't know about?
I ponder this at times, cause sometimes I leave place 'as is', and then return and like an item will be out of place, an item you know you didn't move, would not have moved, had no reason to move, yet when return home, it's out of place.
If it were an intruder, stuff would be gone, valuables would be gone, so I kind of have to rule that out..so that leaves ghost, or squirrel or gigantic insects.
I need to buy some indoor security cameras, than I'd know, like the movie Paranormal Saturday.