Sirius grasped his hand briefly, nodded to Dumbledore, transformed again into the black dog, and ran the length of the room to the door, whose handle he turned with a paw. Then he was gone.
He took the last lew steps at a run, wrenched open the door and almost collided with Mr Weasley, who was standing right outside, looking pale and apprehensive.
Sirius grasped his hand briefly, nodded to Dumbledore, transformed again into the black dog, and ran the length of the room to the door, whose handle he turned with a paw.