It reminds me of a buried receptacle I had to deal with as a teenager.
I had installed it in a frame addition to the customer's trailer on Lake Norman, NC. Days later, he called my father and said that he had paneled the room and the receptacle wasn't there. My father looked at me sternly and informed me of this. I told him that the customer must have been drunk when he did the paneling - how else could he miss the box sticking out a quarter-inch from the studs? My dad just told me to go fix it.
I got on-site and could see the bulge in the wall where the receptacle was hiding. The only problem was that it was "hot" and the breaker box was in the trailer, which was locked. My perenially inebriated assistant still didn't understand that it was simply a matter of judiciously cutting the paneling to reveal the receptacle and then screwing on the cover plate. He looked at the concrete floor and said, "Gonna be mighty hard pulling the wire through that concrete floor!"
I took a keyhole saw and bumped it into the paneling where I guessed was the edge of the box. As luck would have it, it went into the "hot" hole in the receptacle and sent me flying! My assistant exclaimed, "Told you gonna be hard pulling that wire through that concrete floor!"
Ten minutes later, the job was done. A month later, I was off to MIT to study electrical engineering.
The reason the signal was stronger at the box is that the box served as additional radiating surface, ( a bigger antenna), which is why the signal was stronger. This would be the case even if the wire was simply cut off and coiled up in the box. The other reason that it worked in this instance is that the box was at the end of the wire.
in the old days we would just roll all the wires into the box, but later the electrical code required that for the rough-in to be complete the wires would be connected as they would be when the installation was complete except for the wiring device (outlet), so the hash would propagate from box to box, no problem. also a set of plans could locate the box to within a few feet, then use the transistor radio from there. the box location would usually give off a stonger signal due to the ball of wire in the box.
It is clever to use a portable radio with a ferrite loop antenna to detect the brush noise, but I agree the signal would not be significantly stronger at the box than along the wiring leading to it. Perhaps once this method had located the general area, one might use a metal detector, an RF based stud-finder, or even a pocket compass to find the exact spot. All this assumes that the electrician was not so sloppy as the dry-waller and used a traditional steel box.
I agree with Beth, but Bob is right to question why the box was so far back that it could have drywall run right over it and not need a cutout. That is just plain poor work quality. Having done a whole lot of reworks of other peoples installations, it is clear that lazy construction practices make everybody else's work more difficult for many years to follow.
Of course, I am aware that if the box needs a cutout then the sheetrock installer needs to do more work, which does make the whole job take longer, but if the electrician needs to fight with a box that is set back to far, that also takes more time as well.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.