I bet him or his friends live in your building. He was probably just wasted and climbed in the wrong balcony. I bet alot of us have done that in college. I went in the wrong house once and know guys who woke up in a strange house.
This would be my guess too. When I was in college, one of my good friends did this accidentally. Downtown was in walking distance to a neighborhood where a bunch of college students lived (including myself on and off during college). Some friends of ours lived in a house on a street in this neighborhood, and there was a house a couple doors down that looked almost identical. She was drunk one night walking back from downtown and decided to crash on their sofa. Unknowingly she went to the house that was almost identical to her friends house, and the door was locked but the window wasn't, so she climbed through the window and passed out on their couch. The next morning they awoke to find her on their sofa and they called the police. They pressed charges against her for breaking and entering. They knew who she was too from seeing her over at the house that was a couple houses down. Anyway, she got her charge reduced from a felony to a misdeamenor, but she had to do community service and pay a fine.