Something I don't ever see anyone talk about are NPCs in Half-Life 2 (mostly the citizens) when reacting to events in the game. NPCs will actually glance over at other NPCs, aswell as props and even decals such as blood, graffiti, posters, etc. Citizens during the uprising will show signs of being afraid in their facial expressions if they hear gunshots in the distance. Marphy Black, you'd better be writing this down.
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I'll do you one better, Citizens when you pay attention to them can be seen looking at certain points of interest as you've said, it's feature that can be specifically manipulated by the map maker via the ai note hint entities. Node Hint's if you didn't already know are basically a point in the map that tells the ai where they can walk, but the hint part allows for additional functionality, such as telling the ai where they can crouch for cover, and something as simple as, "Hey! That looks cool, look at it every so often." I possibly seen this in action before in the aforementioned uprising levels, specifically rebels seemingly watching the entrances to rooms they're in with their guns up, as well as watching the roofs with that same behavior. Makes sense considering it's a contested tactic utilized by both fighting forces. I'm not entirely sure about dynamic moods such as getting scared upon hearing gunfire. But I do know of the ability to set the mood/facial expressions of NPCs via hammer. It's likely Valve made use of that features at key areas to have a greater effect, such as making Rebels scared in areas where they don't have an advantage.
They indeed change facial expressions sometimes if they hear distant gunfire. I found this out in Garry's Mod (without any NPC altering addons might I add) Where I just had a bunch of invincible NPCs across the map shooting eachother to create a battle ambiance, and I had another NPC on the other side of the map. Sure enough, they showed signs of concern and worry in their face when the gunfire was in hearing range.
Fair enough, I think the hammer setting just sets their general mood then.