As far as single point voltage indicators, this Wiki page gives a description.

https://en.wikipedia.org/wiki/Test_light

It's possible that the measurement van is using the capacitive reactance of the vehicle itself as a coupling method to ground.

There's a LOT of 60Hz a.c. floating around. If a circuit doesn't have equal currents flowing to and from the device wires, it will radiate 60Hz. That's the difference between a transmission line and an antenna at radio frequencies. Given enough amplification, I think 60Hz could be measured/sensed.