Radio direction finding is reasonably straight-forward in theory, but practice is a challenging art.

First, what frequency is the control transmitter on? (or frequency-band/freq-hopping in). If it is one of the usual R/C bands, then that is a limited search space. Or if the perp is not so concerned with observing frequency allocation treaties and regulations, well then now you have a big hunt.

Second, what other signals are in that band? Suppose it is in the 2.4GHz ISM band. Given a spectrum analyzer plot of 300 spread-spectrum signals hopping in the ISM band, which 299 are WiFi and Bluetooth and other, and which is the perp? The microwave ovens are easy to distinguish, but still add clutter.

So if you can pick on a single signal, then you can use Doppler techniques to get a rough bearing, and directional antennas to zero in.

I suspect they had some crews working the problem, but it is a challenge.

Anyone interested can look up Amateur Radio “fox hunting” in the US, and in Europe “radio orienteering” is popular. Since signal hunters are not transmitting, no license is required to participate in a hunt.

>pick on a single signal

You dont have to do it manually. Nowadays you can grab whole 2.4-2.5 range(most common drone remote frequency), process it in real time, at 3 locations simultaneously, and produce signal position map with every transmitter inside the triangle or interest. Even amateur fox hunts can be automated to the point of trivializing whole thing with couple of those https://www.indiegogo.com/projects/kerberossdr-4x-coherent-r... . More professional hardware does exist, quick search gave me http://spectrafold.com/quadrus/product/dru-244a-1-4-pci/ so Im sure MIL already uses this instead of primitive manual techniques.

I know public reporting says US is behind Russia in EW. I don't know how good UK EW is, but if the UK can't locate commercial drones signals, than we are in trouble.

(Fun fact, UK was very good in EW during WWII

https://en.wikipedia.org/wiki/Battle_of_the_Beams )

Military perspective on this:

http://www.thedrive.com/the-war-zone/25608/rogue-drones-have...

DJI drones are very geofenced. To the point where if you try to fly them inside an enclosure near an airport at an officially-sanctioned event like EAA Airventure, you won't be able to.

But many others are not. Drones without GPS for example cannot be geofenced.

It’s not that hard to build a drone as a hobbyist. Controls on commercially available drones are only going to deter the most casual operators.

If my memory of this talk is correct, he demonstrated how easy it was to un-geofence a drone.

https://www.youtube.com/watch?v=5CzURm7OpAA

That assumes it's running factory standard software.