In Stalker its easy to avoid combat and it is easy to run away from combat if you wish, so stealth is not necessary. Sneaking/Stealth is a mechanic that was never implemented in Stalker, and im happy about that, personally im sick and tired of sneaking in games. (I play Misery only, and all of my comments relate to that mod alone. If you only wound an enemy they will usually know exactly which direction from which they were hit and you find yourself in a big firefight straight away. Some supressors work better than others: also the distance from which you engage the enemy can help you take down multiple targets before they can find you. Supressors will often give you the jump on enemies (certainly the first couple you take down) but usually after that, I find I have been spotted and begin to draw fire. I think human enemies will react to sound theywill always approach your last known location and attempt to flank you once you've been spotted, so keeping on the move is highly adviseable if you engage superior numbers in combat alone. I have never yet been spotted by human enemies through walls, but foliage only provides consistent cover if I wear the ghillie suit. The switch distance is to blame, that, plus the fact that if an enemy faces you within the switch range you are almost always able to be seen (the switch distance is the preset range at which the NPC AI reacts to the player and stops its pre-programmed patrol-in my games it is quite long and basically if I can see an enemy without having to use binnoculars, they can see me if they look in my direction and I am exposed). You can sneak around small enemy patrols and even creep up on them from behind or from behind cover, but oftentimes accomplishing tasks requires a mixture of stealth and surprise attack, or stealth and running like hell. Sometimes this works in your favor, however: I was able to sneak into the waste processing station where there is a big group of mercs, steal what I needed and then get out unseen, all because the mecs had spotted me and had all made a beeline to where I was originally (the other side of the compound) while I got around to the other side. Quite often the switch distance will alert enemies to your presence before you are even aware of them, in which case stealth mostly goes out the window unless you can break contact and hide where they didn't see you go.

Stalker call of pripyat enemies