The Banner System
The floating holograms you see of other players are banners. You can find the upgrade to your sentry to place them yourself by going up the ramp next to the guy warming himself at the trashcan after getting a little ways into Jerico City.
How do they work? Well, if you've ever clicked one, you might have seen that you gain some tech scrap (250). That's neat and all, but the real joy and rewards from this mini-sidegame is in hiding them.
See, the more people who locate your banner (in the hour after you post it), the less of a reward you receive. So you want to find a real off-the-beaten-path kind of spot. If nobody finds your banner, you get 5000 scrap, a huge boost to players just starting out!