I've had people stalk me without them knowing tons of times. Well, actually it was just sir guy. But I do see people shoot me out of nowhere perfectly concealed n the shadows. I've done this before with heed's tactical pistol as I snipered people in the shadows without them knowing about it.
For me, when I'm found, I try to use the ramp angles to my advantage building up speed to make a quick getaway into the shadows. This doesn't work all the time as going in a straight line makes me an easy target.