Rabastan was worried about his brother. He knew Rodolphus hadn't really wanted Amycus to come, and now that he was going off to see Bellatrix he knew the animosity could only have grown. He needed to make sure Rod was okay, and to hopefully smooth things over before Am got back.
And besides, he was here to see his brother after all. It would be stupid to come for the weekend and not spend any time alone with him. He had gone by his room, but wasn't too surprised when Rodolphus wasn't there. His brother had never been one for sitting still when he was unhappy.
He's probably pacing around the house, brooding. Or setting traps. Rodolphus had always been fond of traps.
It didn't take Rabastan all that long to find him. He stood at the end of the hallway for a moment, wondering if his brother would notice him, before clearing his throat quietly.