And then you also must consider the natural background rate of radioactivity across that enormous area. Crank your sensitivity up too high to gain range and you might just detect radon or potassium, or whatever else is out there decaying.

Yeah. I would think they'd look for localized "spikes", then follow that up with an on-the-ground search. They don't need to eliminate all false positives.

My idea is to have multiple Geiger counters spread across a 10+ meter pole. Since what they're looking for is small and localized, they could control for varying natural radiation by looking for a spikes that don't affect all the sensors equally.