I strongly belive Minerva knew how Sirius was treated at his home so when they weren't in class she'd talk to him and show interest in his life, in a weird mix between friend and mother, and he just loves it
Mcgonagall: what did you do over the summer Mr. Black?
Sirius, taking out a list: I was hoping you'd ask-
Mcgonagall, whispering: Mr. Black? Are the rumors true? Did Mr. Lupin ask you out?
Sirius: oh my God Minnie yes! I'm s excited, we're going to hogsmade to...
Sirius, holding up two hangers: ok, the black leather jacket with studs on the shoulders or the silver leather jacket with wings on the back?
Mcgonagall, sipping some tea: the black one makes your eyes pop more, but you definitely need a scarf of sorts if you don't want to catch a cold
Sirius: *is crying alone in a hallway*
Mcgonagall: *turns into a cat and curls up on his lap to comfort him*
