Some additional information:

• AI do not seem to register sound at >200 meters distance.
• AI cannot locate players based on sound or direction of sound at >200 meters distance.
• AI only react to tracer fire, and cannot detect the direction the tracers are coming from at >200 meters.
• AI do not react when you kill others standing next to them.
• AI are no longer a challenge for players to kill as long as you stay at least 200 meters away from them, as they cannot find or hear us.
• This bug renders suppressed weapons useless.

Something is terribly wrong..

Update.

Just tried the same test in dev branch. No change.

The AI only reacts when he sees the tracer round fly past his field of view. He then makes no effort to scan or look for targets. After I stop firing for a short time, he gets back up like nothing happened.

Here's a video

I add my +1 here.

My tests yesterday and today, using these conditions (all at once) :

• One red marksman fires over and over at enemies 1km away (without a silencer) ;
• Yet there are two blue soldiers only 110 m away. They never react nor change their stance (clueless no matter all the shots fired).

Those two guards have spotDistance 94 (100 spotDistance with 70 Skill Level).
Of course, the red marksmen is out of sight (only behind a rock), but making tons of loud gunshots only 110 m away.