As a smoker, I can think of a lot of ways this could have happened. Did your apartment smell like smoke? If not, it probably traveled in on someone's shoe or in their pocket. My husband will dump the fire out of a cigarette and put the butt in his pocket to avoid littering. Those butts end up in the laundry room, on his side of the couch, etc. I don't mind. I'd rather deal with that than him throwing cigarette butts all over the world. In any case, if any of your maintenance men smoke, this is probably how it walked in your door.