Snape had reached out for [the map], a horrible expression of dawning comprehension on his face-- (410)
I don't know if Harry quite gets this, but Snape's just realized that he let slip in front of Harry that he has the Mark. Harry doesn't really understand what that means yet, but Snape doesn't know that.
Uh-uh, doesn't work for me. I mean, the way I've always read it was that the look was 'horrible' from Harry's POV: he sees what he's been fearing; he sees Snape understanding that he's there.