Hmmm, strange. I was pretty certain that worked as I saw it written down somewhere, guess not. My bad.
Editing a post makes it appear unread, but doesn't show up in the watchlist.
This is probably the post you were referring to
Editing will make a thread appear as unread with the new icon, but it won't show up in the watchlist. Just a coincidence you noticed it here, probably since I edit my posts right after posting frequently to clarify something, and meta is kind of slow moving, especially early morning.