(snipped)
Or perhaps rudimentary radiation detectors (and possibly sensors of other types, bio or chemical) built into _every_ cell phone: one sensor per phone, randomly allocated, not available to user control.
If the sensor detects a signal above a set threshold, the phone just silently dials in to a collection center computer, reports the fact, then shuts down for a selected dead time.
The collection center computers collect and collate all these reports, and if a sufficient density of reports start showing up in a given area (or along a given track), alerts a human to take a look at the accumulated data, and maybe send a human responder out to look at the general location, or perhaps just auto-query other phones in the same area.
Location to the nearest cell tower ought to be enough for a start; GPS location accuracy not required, at least not initially. The problem of occasional false positives is greatly reduced by having a large ensemble of reporting devices.
If you can build a complete camera into a cell phone for, what is it, about $10 or $20, seems like, with a little development, you should be able to put in just a single rudimentary sensor for the same or less.
If I were to post this same idea to comp.risks, I suspect a lot of potential downsides and unanticipated problems with it would also emerge. Seems worth thinking about nonetheless.