Like someone mentioned, AC vs. DC fields are apples and oranges. An AC field is constantly collapsing and regenerating, this is why transformers work with AC and not with DC. The field is in motion so to speak. You would have to run back and forth through a DC field to get the same effect.

Your home and workplace is surrounded by AC fields, and you are exposed to them over a much greater period than in your car. These handheld devices are no doubt designed to measure AC. Much like a shunt meter you can buy at Home Depot, they may appear to read something when exposed to DC, but it's meaningless.

It gets late early out there. - Yogi