...on a dude ranch with a metal detector as part of an organized trip. After a few false positives, the detector went nuts. We spent 15 minutes digging up a large steel plate with what looked like bullet dents in it (too thick to let bullets go through). Turned out the ranch had hosted a number of movie shoots back in the 40s and 50s, and what we found was a 'gong plate'; if they needed a bullet sound effect, they would set up a mic nearby and blast the metal plate. The ranch took the plate and was going to use it as a sign for their cantina.