I live in a low rise apartment on the second floor and the unit below ours is claiming that our sink or toilet has caused a leakage on her ceiling due to an overflowing toilet or sink. She says that the leakage isn't from a leaking pipe, and has only repaired her ceiling. There has been no history of our sink, toilet or bathtub overflowing, and all of the areas around them are dry. I just don't understand how a leakage can occur without having a leaking pipe. Could it be something else within the floor between us?

Yes, it could be between the floors, it might possibly be a leak around a roof flashing, your toilet wax seal , etc , etc. Her ceiling shouldn't have been repaired until it was determined exactly what leaked. Most bath sink's and bath tubs have overflow holes so that they won't overflow onto the countertop or floor.

There are many possible causes for leakage into her ceiling, and an overflow is the least likely one, since you would have had a wet floor to clean up. It was illogical for her to repair the ceiling without giving you an opportunity to have it inspected since most leaks HAVE TO BE repaire from the lower level.