Hate to break it to you, but Half-Life really stinks.
In an enlightening video, MarphitimusBlackimus digs deep into the original Half-Life's code and discovers that characters don't just remark on smells randomly or because of scripted events; there's actually a whole AI system for it.
In short, there are three scent categories: Meat, carcass and garbage, though it appears that only the first two actually appear in the game. Still, this explains why a security guard will, for instance, comment on the stench emanating from a nearby corpse. It does not, however, explain why scientists won't. They're supposed to, but for some reason, that part of the code is busted.
For Bullsquid enemies, smells mean food, so after a brief period of what can only be deep, nuanced Bullsquid contemplation, they will start chowing down on nearby corpses. Scents can even get Bullsquids' attention from far away, beyond a direct line of sight. After eating, Bullsquids will still seek out things that arose their olfactory glands, but they won't eat any more.
Just like in real life, Half-Life corpses stop emitting scents after 30 seconds.