Okay, this is getting confusing. Let me show you.
Here we have all three windows, "Local", "Region", and "World". I have centered on that single mountain in the middle of the "Region" window, and as you can see there is a small cluster of mountain tiles in the "Local" window. Since the "Region" square is a mountain square, there is a 100% chance that one of those mountain tiles in the "Local" window has HFS in it. I just don't know which one it is.
Every single mountain in the "Region" window will have HFS in one of the "Local" mountain tiles. It can be difficult to find, however, since you have no idea which "Local" mountain tile it will appear in.
I picked a rather easy spot for finding it, since there are so few mountain tiles in the "Local" map. If you were to pick something in one of the mountain ranges on the "Region" map, you'd have quite a few more places where the HFS will show up. And it will only show up in a mountain tile, not in any other. Sorry if this all came off as condescending, I was just trying to emphasis a few points.
To check, I run regional prospector, and I see that the HFS is in one of the outer tiles. I also see that there's a chasm going straight through the thing, another feature that I *think* may also be a 100% probability.
Heh heh. Just noticed that this was "the Land of Yelling".