Surprised no one has mentioned this yet, but when you fight the Nexus Mind, the whole area has holographic projections of Ghosts floating around. Are the Vex tracking the Ghosts? Why? Is there some link between the two factions?

I've found a way to get to the area directly above the Nexus, and got a very close look at the small holograms. They're not Ghosts, though they do appear similar at a distance. I'll be helping someone run their Nightfall later, so I can get a good screencap of them.