OK, here's a mystery. It's Saturday. I see a couple of guys wearing orange vests get out of an ordinary, unmarked car in a residential area. They go to the middle of a quiet intersection, and with a hammer start banging a chisel into the asphalt. They may or may not have hit something hard after a few minutes (change in the sound, maybe). They inspect what they're doing, then brush back in the asphalt with their feet. And drive off. They couldn't have gone deep at all. Some kind of inspection? Outer space aliens implanting homing devices? Any ideas?