Combat advice

New to the stalker games, I really like the gun play and the intensity of the combat. Mostly having some trouble with the other human enemies, it seems like they can start to see me from pretty damn far and I’ll be barely moving. Sometimes I can’t even see them at all while they seem to have a clear sight on me. I figured that’s the point of playing a stalker game tho, either you’re stalking or you’re being stalked. Just wondering if there’s anything I’m doing wrong or if that’s just how it goes with the other humans because I’m getting absolutely lit up sometimes