The way I see it is that what they don't see in their own vision they do see on the overlays; information from other cameras and other agents' vision. That means that, for example, hostiles are marked, their distances known and they can quickly and effectively look at the terrain and get a good idea of their approach and so forth. There's a lot of information to navigate there, similar, to say, the Ghost Recon games. The computers and the networking fills in the gaps and keeps them tracking the same targets. Also, I imagine there is some sort of reticule change for when they do get a clear line of sight, such as for a shot from long distance.
That means they can, on the fly, figure out where the Mullens are likely to cut through with their thralls -- the Mullens themselves are marked differently from the thralls and so forth.
Also, there is a potential police response depending on how noisy the fight gets. Someone might want to address that somewhere down the line. I forgot to mention that Dan's rifle is suppressed, but that means it sounded like a .22 going off. Luckily, not a lot of people around to hear it.
