Well I'm not using it as a HOOK. I'm just "Here's something odd. Players see it, poke at it, then move on."Sure, but I think of it more as fishing than foreshadowing- drop in some bait and see what the players bite on. If they like it, then I'll develop it.
For instance, last session the PCs found a solid circle of black snow. Nothing magical. Nothing overtly done to it. Underneath there was a collapsed hole.
I don't know what it means, I just tossed it out there to be Ominous.