Castiel has a habit of just being there and making his appearance known only when it makes one wonder just how long he's been observing someone who thinks they're relatively alone. In this case, he stands a short distance from Anders, head cocked to one quizzical angle, and asks:

"...You seem different." That could be the peace pipe, Cas. And lets face it, Anders to some degree also seems to be enjoying the festivities, whereas the angel seems to maintain an uneasy distance from it to an almost prissy level -- despite the filthy trenchcoat, he seems to have a dislike of getting mud on him. Or mold. Or sweat or, you know what? This place is pretty much a mess and it's kind of unsettling, so we can leave it at that.