Roaming around the Juarez area (or any other Mexican boarder area) with a metal detector risks a natural perception among interested parties who are not much into hobbies (none that don't involve electrodes, secret compartments, and rapid fire) that the operator is a government agent or contractor scanning for explosive devices and that the appropriate response would be to let him be found impaled on his broken metal detector. After, of course, a long round of implemented interrogation to try to discover what the government is up to since IED's are surely being considered by the cartels, and our hapless hunter is not going to produce any answer satisfactory enough to end the questioning and move on to the killing and beheading part.