When I first moved in, I inherited all the belongings of the couple who had the apartment before me. I presumed this included the two bicycles in front of the apartment, as there had been two of them. It seems that one of the two bicycles had not been theirs, but rather had belonged to the daughter (who had moved out) of one of the other families in the building. The daughter has just now moved back and wants to use her bicycle again.
What puzzles me is why the family didn't say anything about it three years ago when I first started using the bicycle. If they had put a note or something on it saying "stop using my bike" I would have realized the error long ago. <shrug> I guess maybe they were too shy...?