"You don't want to know," Severus assures him dryly. It's a good memory, actually, but he knows Q wouldn't appreciate it, the way he feels about Albus. "Thank you, no," he blinks at the force of Q's sympathy. "I think they're better gone; I can accept the occasional moment of uncertainty."
"Yes," he agrees. "If your first scenario is correct, there's nothing to be done about it and, assuming your battery of tests was complete save this one, no way to tell. In any of the others, the presence or absence of a possessing entity is unlikely to affect either of us in future unless one happens to take notes in the expected language, and since that possibility is easily avoided, I believe we may now proceed to treat it as nothing other than a book." He slides Q a puzzled glance about the preservation.