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.