I feel like this game was designed to be a third person shooter. Those who play on third person get a huge advantage over players who play in first person.
There is no possible way to sneak around enemy AI in this game only other than in third person view, using the method, "I can see you from behind cover because the camera is further back than my character so I am hidden but can still see you".
If you try sneaking in first person, the moment the AI is facing you in any manner, they immediately spot you. Even if you are close to a thousand meters away and laying prone with camouflage in foliage.
The solution is to make the AI less like robots with their thermal like vision in detecting enemy.
Add a Field of Vision Cone for each AI. And if you are in their FOV cone, they then will register that you are there. If you are really close proximity to them, they will spot you quickly. If you are very far away from them, they will fix their FOV cone on you, but it will take them longer to determine if you are enemy relative to how far you are. They can also bring up their optics or binoculars to determine if you are enemy if you are at extreme ranges.
Close-range from enemy:
Enemy spots you in their FOV cone.
Enemy opens fire on you.
Mid-Far ranges from enemy:
Enemy spots you in their FOV cone while you were doing something very incorrect. You decide to stop moving and go prone. (If you are moving for them long enough for them to register you as enemy, this does not apply)
Enemy stops moving and looks at you doing a little look animation, (Head going forward a bit, eyes squinting, etc)
250-300 meters is 80% chance they will spot you and register you as threat.
300-400 meters is 70% chance they will spot you and register you as threat.
400-500 meters is 60% chance they will spot you and register you as threat.
No matter what range: Enemies have a 60 percent chance of taking out their optics or binoculars to spot you if they suspect you are within their FOV cone.
Obviously these conditions would change if the enemy has recently engaged enemy, their alert levels would be even more high, so a alerted enemy will use their binoculars or optics 100% all the time if they see movement in the FOV cone at extreme ranges. Their recognition of targets will also be higher if they are in extreme high alert. The level of alertness depends on their situation, if engaged in combat, discovered dead bodies of friendlies, etc...
The stats in this are for example only and are in no manner the correct ranges or time responses that are optimal for immersion.
I propose that the enemy sometimes look at you if you are really far away and disregard you are not enemy.
Movement is what attracts the eye, if the enemy looks at you from really far distance, and you do not move, there is no longer movement in their eyes. They may disregard you as part of the landscape.
If you are watching the enemy thought binoculars, you can see their reaction telling them if you have been discovered or not. There will be indications, such as the enemy pointing their hand towards you, or they decide to move fast all the sudden, move as a unit, take aim and fire at you, or take cover from you.
Also if you are in camouflage in foliage, in close or medium ranges and the enemy suspects you may be a threat, the enemy may walk closer to you if they suspect something, when they get closer, they will recognize you as enemy.
Majority of characters in this game use camouflage. This AI behavior would make sense, because it is harder to spot targets if they are not moving at far distances. If they are using binoculars are optics to look at you, they should recognize you are enemy immediately.
If you are not moving and far away from the enemy, using foliage and have camouflage. The enemy should not spot you when they turn and look at you.
Far away from enemy should grant you most security in being spotted, the closer you are, the more likely they will spot you.
Also AI does not respond to smoke grenades, I try and use smoke grenades as a distraction in the showcase "Steal a Car". But the enemy AI does't register anything strange as white smoke appearing on one part of their base to go and investigate.
It would be really nice if the AI had a behavior with different alert modes.
AI have weapons down, or holstered and are going about their business, chatting, smoking, sitting down, leaning on objects, cleaning equipment, loading and unloading equipment, etc.
AI have weapons down, are walking around perimeter, making sure nothing is going to enter their compound or area of patrol.
Search and Destroy Patrol:
AI have weapons up and ready, are searching every part of their compound or area of patrol, covering every inch and hunting activity for you or other enemies.
No explanation needed.