Good point, having been fired upon in real life, what alerted me to the location or source of the attack, was muzzle flash, then my eyes zoomed in on the attacker. Now granted some aliens have better eye sight or better sighting technologies, and some may have inferior. Likewise player sighting equipment or detection devices (motion sensor for example) would be helpful as well.
Wonderful topic LoS, it has so many dimensions to consider, like my simple peripheral vision thread and the counter considerations provided. It is a game, and it cannot be a perfect real life simulation.
Sometimes I have the LoS advantage and sometimes I do not. As Gunny Highway says, you adapt, improvise and overcome.