At first, Derek hadn't even noticed that Stiles was avoiding him. Allison moving into the house had meant that Isaac had moved in with him, and Derek was still dealing with the Argent reveal and catching Isaac up, and feeling guilty for having kept the secret from Isaac to begin with. He was taking advantage of their shared living space to pay more attention to Isaac to make up for it, which meant rather a lot of sex.
So it was only after the snow melted that Derek realized Stiles was nowhere to be found, and then it became a point to see just how long it would be before the human sought him out. When it became obvious that Stiles wasn't going to seek him out, Derek decided to seek him out instead.
He found him in the laundry room, obviously enjoying himself, so at least the human hadn't been neglecting his chores (and with all the snow and wet from outside, there had definitely been an increase in laundry). He'd just been neglecting Derek.
The werewolf said nothing, leaning against the doorjamb with his arms crossed over his chest, studying his wayward slave with a critical gaze.