Who / How Can A Drone Be Detected?

With drones becoming more prominent, they increasingly become a security risk. The most obvious example is airplanes / airports but many high security facilities are at risk to drones conducting spying / intrusions into their space.

What can be done?

The only company I have seen so far claim to detect drones is SpotterRF, see video below:

Anyone have any other ideas or experience in this area?

Drone Defence - However, I can say nothing about them.

Saw this at ISC West but I didn't get any details.

DroneShield is apparently doing a $5M IPO offering in Australia, they also have an office in the US.

Add Dedrone to the list, a US startup that got $10 million VC funding in May 2016.