Is the student union also known as the dorms? In that case, maybe he was a squatter...living in an empty dorm?
The student union is the student union, not dorms. The unions usually have couches, game tables, snack bars,TVs, etc. Nick must have been sneaking in, or staying in after hours & camping out on one of the couches. He was still a squatter.
Two different things--dorms are where students live; student unions are more like "lobbies" at a hotel (for want of a better description).