When walking by co-workers, I can't, like a solitary hunter, ignore their presence, or make a threat display. Though that would be pretty great, establishing dominance in hallway exchanges on the way to the can. "Hi, David." "GRRRRR!" (baring of fangs, lesser nerd shows soft white underbelly.)
But no, what happens is a strange tightening of the mouth. It's like a smile, but no teeth are shown, and it's mirthless and unfriendly. It's a simple acknowledgment of the actual presence of another person, but without any sort of actual communication occurring other than that. It's like a "Oh, look, you are there and I can't ignore you" grimace.
Why do we do this? We're such weird animals.