“I see,” said Professor Quirrell. “And what would you have done about the threat to me if your spell hadn’t worked for destroying the Dementor?”
“Plan B,” said Harry. “Encase the Dementor in dense metal with a high melting point, probably tungsten, drop it into an active volcano, and hope it ends up inside Earth’s mantle. Ah, the whole planet is filled with molten lava under its surface—”
“Yes,” said Professor Quirrell. “I know.” The Defense Professor was wearing a very odd smile. “I really should have thought of that myself, all things considered.”
(...)
“Oh?” said Professor Quirrell. “But there is an interesting pattern to them, you see. One might say it sounds like something of a riddle.”
What Quirrell is remarking on is that Harry’s following an elemental pattern, not that he himself followed the same one.
Q is saying that he didn’t think of that as a thing to do with a Dementor and should have. One possible reason for his “very odd smile” is that he did already think of it as a thing to do with a horcrux. I think it’s reasonable to call this a hint that maybe there is a Voldemort horcrux buried deep underground. (Certainly far from a proof that there is, though.)
(...)
What Quirrell is remarking on is that Harry’s following an elemental pattern, not that he himself followed the same one.
Q is saying that he didn’t think of that as a thing to do with a Dementor and should have. One possible reason for his “very odd smile” is that he did already think of it as a thing to do with a horcrux. I think it’s reasonable to call this a hint that maybe there is a Voldemort horcrux buried deep underground. (Certainly far from a proof that there is, though.)