"It was a voluntary departure, actually," he said, his voice deadpan. He tried not to wonder what had become of the collections in his office. "When the task your son was given had been completed, I decided it would be unwise to return to the school. - And then, the Dark Lord has no further need for someone to spy on Albus Dumbledore."
Snape was unsure how well informed Narcissa's sister and husband kept her, and was inclined to be a little vague. "As much as I would like to return, I doubt it would be very productive, at the moment. - How is Draco, by the way?" As though the boy weren't the reason he was here in the first place.