I just noticed that I have a copy of Scratches installed, despite the fact that I've never purchased the game, it doesn't show up on my account, and I don't remember installing it. To make things weirder it's installed in the start menu's Games folder, which I never install to. All of my gog games go in a user library. I'm terribly confused by this, and the gog.com contact forms didn't really have any options here which were applicable to my situation.

You, sufficiently long ago to forget about it, or someone else who has access to your computer, has already bought or pirated this game and installed it.

Check the files dates. Or maybe hints in some readme's (or whether it's the director's cut, etc).

:-/

Well now that you have just enjoy it :P it's an excellent game after all