Maybe you're approaching them at too low distance? AI can hear too, and your movement affects a lot to it. On the other hand, if they spot you once, they will keep tracking you and if they loose visual contact, they'll try to estimate your movement and location until they spot you again.

Here's really good quote by armapirx:

"Arma3 AI doesn't see through bushes and trees' foliage. Tests in editor showed that. But the AI is very good at keeping track of player's position and movement exactly in a similar way, as we have slowly vanishing symbols of enemies on the map (with the option extended map info enabled). It's only that AI's tracking is better and more accurate, than those symbols are.

I'm not sure about the grass. While laying prone appears to significantly increase camouflage factor, the grass itself doesn't appear to block AI's view. You can crawl through a grass close enough for a pistol kill, but once you've been spotted, it's game over.

Still, by using tactical movement and changing position frequently, it's possible to confuse the AI about where you are and to flank them. Laying behind a single tree and picking them off doesn't work and honestly that's how it should be. It's either flank the AI or they will flank you.

AI doesn't see behind its arc of vision. It's possible to approach very close to them, until they actually hear you and only then turn at you. Based on my observations and tests, I think that "seeing" through walls is rather orienting towards heard sound. Try sneaking on them with slow movement and see the difference.

Stealth approach is very usefeul, when setting up for an attack or performing recon. But after "going loud", you'd better be on the move! AI is hard to beat and smart, but in tests designed to check it's capabilities and used skills one by one, it appears that it doesn't cheat. I do suffer from the situations, when I'm being fired at from behind the trees, but I do the same, when I have a suspicion or report that the AI might be or is hiding there. Actually I'm worse, because I throw grenades blindly, just in case, while AI doesn't.

I'm not sure about effectiveness of the silencers in Arma3. It appears, that they do have some value, but I haven't played with them enough to say anything for sure. As far as real life goes, silencers don't make you absolutely silent and undetectable, so don't expect shooting anyone in a room, without alarming the guys behind the wall. A silencer may delay finding you, if used from a distance, but they will find you eventually, unless you change the position."

Yeah, I kinda agree with OP on this.
Even when I in the Editor took away E_AI nightvision, set the game to 0015 and was crawling slowly at 50m they instantly saw me and started shooting.

Compared to A2 sneaking around in A3 regardless of setting seems impossible in comparison.

Same issue as #4612. The AI can hear you from too far away, even if you are sneaking around.