even then, they're still bound by the 20-tile visible distance rule. (apart from psionic or blaster launcher stuff) it may just be something to do with reaction fire taking place, and killing the unit "before" he exposes the alien, maybe. or maybe it's an alien inside a building that makes it juuuust dark enough to obscure the alien, it could be just about anything to be honest. but yeah, i do remember taking fire from invisible aliens in the original in this regard.