Snape arrived at Harry's rooms - he wasn't certain Harry was there, but it was a good place to start looking. If he wasn't there, Snape would go to the farmhouse and talk to Dumbledore.
It was all he could do to force himself to arrive outside the door and knock. Whatever regard he ever had for the social niceties tended to go out of the window in times of crisis, but he really would rather not alienate Harry any more than absolutely necessary - the whole issue of Al was more than enough of a bone of contention between them already.
So, he knocked, and waited, though every instant galled him.