I think it is pretty clear from the pole example that they are not picking up arcing. What is mounted on the truck seems to be just a very sensitive non-contact voltage tester which is tuned to 60Hz. If the energized surface was just, for example, the small handhole cover plate it would be less sensitive. Live wires not shielded within a raceway would also be picked up (the overhead mentioned), while service wiring with a concentric neutral acting as a shield or an L1-L2 pair with balanced voltage would not show up at that distance.