Well, this one is simple (the problem, that is): enemies don’t get alerted by finding the dead bodies of their comrades (killed stealthily). I even managed to kill dudes that were exactly one step away from their mates’ field of view and got away with it. It feels off.
Now, considering this game’s difficulty, this shouldn’t just be a simple fix, but one accompanied by a new game-play mechanic, which could be one of two (or both):
A “draw attention” mechanic (like whistling or noise making), which would make an enemy leave his post and get killed in a more hidden position.
A body hiding mechanic. It could be implemented like in Phantom Doctrine (watch the body pickup animation - screen goes black - screen lights up and the body is gone) in order to simplify the game’s coding and not have to introduce actual hiding places for the corpses.