I posted this in another thread a while back but here goes:
This happened to a friend of mines house mate during their time at university. It was at the house she lived in before moving in with my friend after this event occurred.
During her second year at university she lived in a house with three other people. At Easter, everyone went back home to their parents for the Easter holidays. Her first night back after returning from the Easter break she woke up in the morning to find some things moved about in her bedroom. She thought nothing of it and put it down to moving it herself the night before and not remembering. She woke up the next morning to more stuff moved around. Like her TV and chair etc. she started to get a bit scared and told her house mates to quit moving her stuff about as it wasn't funny and very creepy. They all said they didn't touch anything. The next morning even more stuff had been moved and she started to get very worried. Again, she told her house mate to stop but they swore they haven't touched anything and were getting very concerned themselves. The fourth night she woke up during the middle of the night. She looked over and in the darkness she could see a figure sitting at her desk. She freaked the fuck out, ran out of the room, locked the door and called the police. The police turn up and they unlock the door. Sure enough, there is a guy there, probably early 20s, sitting at her desk. Calm as anything. The police arrest him and take him away.
A couple of days later they find out that the guy was actually a mentally handicapped person who lived at a care home not far down the road. His mother who was his full time carer had looked after him until her death a year or so earlier when he was then moved into the home. He's been a missing person for a few weeks now and it turned out that the house this girl and her friends lived in used to be him and his mothers house. In particular, her bedroom used to be his.
He eventually told the police that when he ran away from the home, his old house was where he went to. He managed to get in somehow and spent the whole time there.
Two and a half weeks. Sleeping under the girls bed. Coming out at night while she slept to move her stuff so it was like how he had it when it was his bedroom. Safe to say, she moved straight out of there and that's when she moved in with my friend.