you'll get a small indicator letting you know that there's an enemy in your peripheral vision
Is it only me, who thinks it looks like radar cheat?
How in real life your peripheral vision is able to detect enemy?
In reality, your peripheral vision can detect movement and can guess color of an object.
So suggestion is, to change peripheral markers to "peripheral vision line", which will contain color of object at your peripheral vision.
Based on that color you could tell, what is happening in your peripheral vision. If you see color change - that means something is moving there. You will need to turn to check if its enemy or not.
Hope it is possible with UE3.