@Alwarren
"Why is this a bug that needs to be fixed? Because it makes stealth based missions pretty much moot. It is no longer a question of situation whether you can risk taking out a guy or not, you will have to ask yourself, is he grouped or not? Because if he is grouped, it doesn't matter where his buddies are, they will know."
It's actually even worse than this. The AI does not have to be grouped for this to be a problem. The AI only has to have seen one of their guys in order to become completely aware of it (i.e. know exactly when it dies by enemy fire).
So for example. If the enemy has setup a roadblock and one of their patrols walk by, you can't eliminate the guards in the roadblock without alerting the patrol. Even if the patrol is beyond visual/audio range.