The sound of a loud engine caught Regulus' attention even through the heavy warding he'd added after Bellatrix's "present" had been delivered, bringing him out of the textbook he'd been reading on Animagus magic. A knock on the door made him jump, startled and apprehensive, and his heart was pounding erratically as he went to peer out the peek hole to see who was there.
Sighing with relief, he unlocked the door and opened it, regarding Sirius with a questioning look. "This is quite a surprise," he said, standing aside and gesturing a silent invitation for Sirius to come in. "What brings you by?"