An MIT astronomer famously explained why aliens haven't contacted us yet.
There is software that can track drones in open areas, but none that can do so in tight-knit, urban ones.
Imagine if you will a terrorist cell getting their hands on a drone, or a number of them, and some explosives. No, this is not an episode of 24. The military calls drones unmanned aerial systems (UAS), and they are worried about them. While planes and helicopters are large enough to recognize easily on radar, drones, being smaller, can be mistaken for birds.