Alright, how about exclamation marks:
I figure it's the same concept as the skull, in that it's a harder level? I just did that one with three other dudes who were way more advanced than anyone else I've seen. They tore through it like it was nothing, while I gained 4 levels in about 10 minutes.
Those are alerts, special missions that aren't necessarily harder but can feature special rewards for their completion. High credits usually, blueprints occasionally.