Enemies' spawn mechanics.

[Akkensheen]

I like this game and have a good time with it. Until I tried custom difficulty between Hard and Extreme. Then I saw an NPC spawned out of thin air. I think I was lucky due to using the x6 ACOG and going out of the scope the moment I moved it ~45 degrees.
But then I got a similar case in Bunker. I was looking at the door, then looked to the side, and looked back at the door, and saw a surprised NPC 2m away from me. I didn't hear it, which is weird since you can hear NPCs from behind a walls. So I killed it, waited for a couple of seconds, and repeated the look at the door, then to the side and back at the door. And to no surprise of mine, there was an NPC pointing a gun at me, all in complete silence and no shouting, no footsteps. This time it got me.
This is unacceptable for the type of game it tries to be. Since players have an assumption that the enemies have a fixed number and can't spawn behind their backs, or for that matter, 2 m from them in a blind zone. I just hope this is a bug and not intended mechanics to make the game more intense/hard.

[royrockbear]

Artificial intelligence can also act silently.
Especially when it hears your voice, waits for you somewhere, and suddenly appears around the corner to meet you.

[Akkensheen]

Nope, I checked the room before, and the only sound I made was opening the door to evac in the Bunker. So no one can wait for me there, and they have to make steps sound since they appeared in a 0.5-second window; they must be running => sound of running

[royrockbear]

I've seen AI creep up on my teammates in co-op, and I can assure you, they are very quiet.