"He had no reason to tell you anything. He did, however, tell me when you came to him to warn him of the Dark Lord's plans for him. It was not necessary to give you any other information then," Severus pressed on, watching very closely - half hoping he wouldn't have to waste time on further denials, and half expecting it from anyone quite as closed as Draco had proven to be. "Now it is. When I return to the school I expect to be in a very visible position. I will need someone to help me continue Dumbledore's work, and you seem likely enough to evade suspicion, if you're careful." And there was a convenient threat available, of course, if he refused. That helped.