Well, lets see here. Now if I had been at any time an official "Spook", having anything whatsoever to do with the "three letter" Agencies, here's what I would do to see if there were "drones" within a several mile radius of a certain location...
A: Listen for Drone air to ground transmissions by doing sweeps with highly sensitive and directional antenna arrays on both the 2.4 and 5.8 ghz bands. Listen for the signature signal combining telemetry, control, and video transmissions. QED (quite easily done).
B: Having acquired this fairly generic "signature" signal, I would then look for a unique identifier of that aircraft if any (I'm not talking the "flying license plate proposed by DJI, but it is unknown if there may be an imbedded code in each UAV, but if there is, I don't have much doubt that some arrangement may exist so that the "Agency" may query DJI and see who the craft is registered to...
C: Gee, I almost forgot...I'd also watch the video stream too and probably find interesting clues as to location, and given the penchant for "selfies" these days, also view the pilots smiling face either departing or returning home, AND since the cameras on our UAVs are pretty good these days AND there are now large databases of "Faces" from our "hey look at me" culture and marvelous "Facial Recognition" software available...
...I don't know why I have a suspicion that in about 30 minutes after posting this I'll get a knock at the door

I hope its Mulder and Sculley this time, I sure do miss em'
