It probably found it's way back and you probably already had two anyway. People need to give more credit to little animals, they are more able than you expect! You need to get rid of the little micies before the problem escalates and you end up with other unwanted 'pets' or in the back of one of you're colleges vehicles with some nasty disease caused by mice poo. (Mind you I'm sure they would love it and give you a nice new nick name). When I was a student we used a humaine trap then dispatched them in a bucket of water (Well it was all we had, and as an added plus we didn't have to scrape up mice eyes from the other side of the room). All apart from that one that hid behind the microwave, a gentle tap from a large spanner saw the end to that one. They didn't come back either.