I'm sure I'm not the only one who's noticed this. Often just over the horizon, most often when looking up at steep terrain, you can see some sort of "phantom" terrain that slowly disappears as you walk towards it. My guess was that it's a coarse terrain map that is drawn outside the terrain view distance but adjusting the terrain distance didn't affect its appearance. I'm still guessing it's somehow related to coarse terrain.

I can upload screenshots and/or video if necessary.