It could be that someone forgot to mark your room as rented, shift changes, new front desk clerk sells a walk-in customer an "empty room", and whoops! Hopefully, it was that innocent. Certainly a possibility .
Discussion
Yes it is also a possibility. I think it might have been someone not so lucid who gave the wrong room number at the reception asking to have a copy of the card and found out it was the wrong number when he entered the room.