Some NPCs have squad behaviour. I can't remember all of them, but I know that Human Grunts, Alien Slaves and Houndeyes definitely all use it.
Basically you give, say, 4 human grunts a squad name of "alpha". Any members of Alpha squad will report to the other members of the squad if they see the player, alerting them all.
Super cool and mostly unknown behaviour of houndeyes is their shockwaves actually
grow more powerful depending on how many of their squadmates are nearby. The power of the shockwave is shown by its deeper colour and increased damage output.
Also, in an unalerted squad,
houndeyes will fall asleep, leaving one on guard. If alone, houndeyes will stay alert.
There's all sorts of cool AI behaviour most people don't know about in HL. You'd be amazed
how clever cockroaches are