I just encountered Hugin with a message that I was not able to find anywhere on the Internet, including the wiki. See the attached image.

I went into the cave anyway, and it turned out to be Howling Caverns with Geirrhafa inside of it. I have not taken any quests from Hildir yet, but I did locate her already.

I though about adding it to the wiki, but I'm not sure how it should be described, and whether my guess is actually correct. In particular, I'm guessing that this message appears when approaching any Hildir's quest location.