A brief and terrible magic lashed out from the Defense Professor's wand, scouring the hole in the wall, scarring the huge chunk of metal that lay in the room's midst; as Harry had requested, saying that the method he'd used might identify him.

Chapter 58

I'm kind of worried about this... all the real attempted solutions I've seen use partial transfiguration. But if we take "the antagonist is smart" seriously, and given the precedent for V remembering and connecting obscure things (e.g. the Resurrection Stone), we should assume V has protections against that tactic. It is not a power the Dark Lord knows not. And come to think of it, V also saw Harry cutting trees down with partial transfiguration.