His last interaction with the boy wasn't so great, and he wasn't eager to pretend to make friends again just to pick up his stuff. Instead, he opted for breaking in.
What other choice did he have?
The bluenette teen approached one of the windows of the home from the outside and started to pry it open, but it took several attempts before he was successful - and unfortunately created a great deal of noise. It was late at night, so he was hoping everyone was asleep and wouldn't have heard him - or hear him as he crept started to climb through the window to search the house.
Sleet Tempest Snape