Even though Derek had to remind Castiel to use the door like a person and not just pop into someone's bedroom when they're about to get laid, he still agrees to the rather odd invitation. This is Castiel; most things that he does are rather odd.

Unfortunately, Barry can't come because of superhero related things so Derek shows up to the house solo. He goes around back, following the scent of Dean and grilling meat. He goes straight to the grill and nods in greeting at Dean, and then looks over at Noah in faint surprise.

"It really freaks me out that I can't always smell you," Derek says in lieu of a hello, and then looks at Dean again. "Why is Castiel not fully clothed? I hope it's not that kind of party."