Rage has plenty of opportunities to attack enemies that are unaware of your presence. Consider the following situation:
It would be very easy for me to dispose of the gentleman sweeping the floor in one of several ways. However, I am unaware of a way to go about it without making the other enemies in the room aware of my presence. A shotgun blast to the back of the head would definitely do the trick, but would obviously be too noisy. Trying to be more subtle and relying on my fists doesn't cut it either, as I would need to get too close to my target and he would end up noticing me.
Is there some sort of trick to it that I am not thinking about, or perhaps a suitable weapon that you get later in the game?