Try a few times my repro missions. Set a medium level of AI skills and hide in the bushes and shoot at the enemy. The distance to the enemy > 400 meters. You will be detected within approximately 5-10 seconds! I think in real life, at a this distance, it is impossible to detect so quickly, even weapons without sound supressor!
The second mission demonstrates the same problem in bad weather. In the fog and rain, the player will be detected also quickly. This proves that the bad weather does not affect the AI detection or this detail poorly implemented in the game.
Try to use a pistol with a suppressor in my repro missions. This weapon with a sound suppressor should have the least detection-effect in the game, but the enemy AI also will easily and quickly find your position! Also try to use Kir with my fire position. As I know, this weapon has subsonic ammo and should have a difficulty for enemy detection, but it also does not give any result.
This suggests that the AI-detection in the A3, not consider the current and specific weapons characteristics.