During a remodel, a drywall subcontractor covered one of the outlet boxes that had TV coax. It was flush enough to the stud that they could not find a "bulge" with a long level. It was somewhere in a few feet of the wall, at a known height, but they didn't want to "just start cutting" because of all the patchwork that might result.
My solution was to turn off the breakers that serviced that portion of the house. Then I connected the other end of the coax shield and center conductor together and tied them to one phase of the incoming 240VAC two-phase power. This meant that coax was the only thing in that wall with AC. Then I handed them an electrician's electric field tester - available for a few dollars at any hardware store - and had them use that to find the now "hot" coax. They found the field - and where it stopped - in seconds. I disconnected the coax, they drilled a pilot hole which went right into the box, and finished with a RotoZip to open up the rectangle. Presto... the box was exposed with zero additional drywall damage.
An electric field probe can be a very handy device for determining the presence or absence of an electric field if you have a conductor available to carry it. Even if only one end of the conductor is accessible!
Disclaimers: I am NOT advocating anyone do this. I am NOT advocating that anyone try this with a dog fence wire. I AM advocating that you avoid mains power, hire licensed and bonded professionals, take out a large umbrella liability insurance policy, and not be on the premises when they're working. YMMV.