All kinds of ideas. Unfortunately, it would just be noise, in a sea of more noise. And.. if anything ever did come of it, it would be better, and more effectively deployed at the airport.
Drones are too small for radar. There fore, it seems the only way to detect them, would be to listen for them on 2.4, 5.8, and 900 mHz. But, there are already so many other applications in use, within this portion of the radio spectrum. Unless you could filter everything out, but drone commands, and I don't think that's possible...to differentiate between RC telemetry, and everything else. I think it would actually be easier to listen for the air disturbance that 4-10 inch props make, in the atmosphere. Probably the most cost effective approach would be just spotting them, visually. Either with cameras, or binoculars. Perhaps biometric software could be employed, to quickly scan ahead, for anything the basic size, and shape, of a drone. Then, again....that probably wouldn't give much lead time, from an aircrafts perspective. Probably better employed at the airport, again.
But, I don't see much of a future for the technology, even if it was invented. More birds have caused plane crashes, than drones ever will.....and nobody seems to worried about rushing into technology to prevent that. And it would be a simpler feat to accomplish.