"You should certainly ask. I'm sure Professor Slughorn is just as keen to get his notes back as anyone. But of course, extracting anything from someone's brain is best done with consent. You're not going to find anything here. Whoever did this was extremely thorough."
He had some suspicions about what was going on, but he kept them to himself. It didn't take a genius to work out one or two people in who's interest it was that the quarantine stay in place. "If you speak to Mrs. Malfoy, best get her alone if possible. Better still, have a Slytherin ask her if you know any. Might as well err on the side of caution."