I think the number probably got stolen when I was having lunch at Subway across from the U of W back during exams. The transaction didn't complete the first try, and the machine spit out an error message with the full card number on it, and I didn't think to take that receipt. There were some seedy people there that day (trying to steal potato chips of all things); they probably picked it up off the counter when nobody was looking.
Oh well, it's no bother. They've already mailed out a new card.