There’s an add-on feature for WordPress that I’ve played with a few times that extends searching to include comments. But it has a couple of quirks. It adds an index that’s seen as an error by the database, and it prevents me from searching posts when I’m in the editor. Anyway, it’s enabled right now, so if you give it a try you’ll probably see it brings up posts that don’t appear to have the thing you were looking for. You’ll undoubtedly find it in the comments.