They were found through quotes from Brutus as you completed different things during the map. Weasel's number was found when somebody got the free Blundergat from the Wardens Office and they heard Brutus say something (I do not remember this quote). Another one is heard when you get the Golden Spork from the Cafeteria, something along the lines of "Your number is up 872B!".
EDIT: I remember the quote from the Wardens Office; "I'm coming for you 872B/101A/481B/386A!" (It's a random number each time apparently).