Most drone controllers sold on the market today already have a built in feature to detect loss of signal between the receiver on the craft and the transmitter.

Most drone controllers sold on the market today already have a built in feature to detect loss of signal between the receiver on the craft and the transmitter. It's usually called Fly Away Prevention or failsafe and the detection method is pretty simple. I think the major issue with having RF seeking drones is the direction finding itself. Being able to do that cheaply would probably require a fairly expensive antenna array and SDR to do the processing.