Why not have a combination of enemies appearing on the radar when targeted, and them appearing on the radar when they shoot?
I dunno, I just like the idea of having ambushes like that. You know, you're driving along, not noticing the bunch of rocket soldiers hidden in one spot. Your radar's clear, all you see is yourself, and some other guy in the distance. Then, all of a sudden, it's all "OMG RED DOTS EVERYWHERE". Then you get hit with 10 rockets.
Or, well, gold dots if you're Nod getting ambushed.
But still, the system functions in identifying immediate threats who are trying to kill you, but still rewards people who are stealthy, as well as people who are perceptive.