One particular problem area, which admittedly may be a bit of an anomaly, is that first little area early on in Contact on the beach where you have to take out the jamming device (4 guards on the ground and a couple of boats circling in the water). Generally speaking, that entire hill seems to be a pretty funky area stealth wise. Often, even if you can't be seen by anyone, the target can't be seen by anyone, and you're well out of earshot of the other guards, somehow the other guards still instantly know when you shoot him; they turn yellow on radar and move to shoot off the flares.
However, the thing that makes me even more confident that the mechanics are faulty (at least in this part) is the contradictions. For example, when I'm proning on a rock near the bottom of the hill, completely undetected, and about to kill the near, right guard. All of the other guards are pretty far away and not even looking. If I take out my Pistol (silenced), cloak, press the crouch button to rise up off the ground, and shoot the guard, everyone suddenly becomes alerted. If I use the SCAR (silenced) instead, no one becomes aware. You can do these trials side by side in exactly the same way and these will be the results consistently. I have the save file if anyone wants me to send it to them so they can try it out (it wouldn't let me attach it), unless there's something that would prevent someone from playing my save file--not sure on that. I was just wondering if anyone else has experienced similar things (the game has been extremely buggy for me at some parts) or if anyone has an explanation for this. I also just wanted to point this out in the hopes that such issues could be avoided in Crysis 2. Thanks.
-Pepe Silvia