This happened to a friend of mine - she told me about it a year or so ago. We'll call her Minji.
Minji is in her late 20s, and works as an English tutor in South Korea. One evening, a few years ago, she was tutoring a high school boy. They were up studying pretty late, and the buses stopped running. Being a long way from his house, the boy asked if he could crash on her floor overnight and get the first bus the next morning.
Minji was very reluctant, because inviting a teenage male student to stay the night didn't sound like a great idea, but he was begging her and eventually she relented. They went back to her one room apartment, and she got into the bed, while he laid a blanket out on the floor, and they both fell asleep.
A few hours later, at maybe 2am, the boy wakes Minji up. "I'm really hungry", he says, "let's go get some food". Minji opens her eyes and looks up at him in disbelief. "Food? Now? It's 2am, go back to bed." But the student insists, "no, I'm so hungry, let's eat something now." She tells him that there's some ramen in the kitchen, and he can fix himself some. This doesn't satisfy him - he doesn't want ramen, there's a 24 hour place just down the road, let's go there.
Eventually, after several minutes of persuasion, the boy gets Minji to come with him to the restaurant. They leave the apartment and head out. As soon as they're on the street, the boy turns to Minji, and says "I'm not hungry. I woke up in the middle of the night, and looked under your bed. There's a man sleeping there."
They call the police, and discover that a homeless man had been living in Minji's apartment, sleeping under her bed, for over 2 months. The boy only saw him because he was lying on her floor, so had a clear view under the bed.
The police arrested the man, and thankfully there were no other issues, but that's by far the creepiest thing that's ever happened to anyone I know.