So...detected my friends house. At pro was constantly erratic.....could this be due to the fact that for 50 years there was a nearby coal burning plant? Yes, i did ground balance....reading was 81.
If your search site is where the old plant stood, it is likely that the ground is so full of "junk" from the demolition of the plant that you would get a nearly constant signal. If your site is a distance from the old plant, it is not likely that anything there would affect your detector. Another if ... if you are near enough to the plant and they did not completely remove the ground grid, you could be picking up stray induced electrical signals.
The situation is too complex with too many variables to offer a simple answer.