I have seen some programs where the investigator is using a "Trifield" gaussmeter, a brand that is known for inaccurate readings (I lab tested one extensively).
However, it is sensitive and the needle does move in a magnetic field. When they get all excited in some attic or basement when they see the needle move, I would assume they are near some wiring, often knob and tube in old houses. So I take it as show biz, not science.
One producer bought an electric field meter from me when I was manufacturing them, and then sent me the raw footage of their investigation, which showed the digital display suddenly jumping up. I ran it a few times and noticed that the guy was subtly allowing his thumb to touch the E-field sensor, resulting in the spikes. They weren't happy with my evaluation.
I have done a lot of reading about survival of consciousness as well as the ones who hang around after death, and I have no doubt of their reality. But I have yet to see a TV program which is legit. They may have some valid footage infra-red, etc) but their motives are to sell their show.
Karl