Fluctuations in background radiation.

Changes in resistance of the ground, especially if around powerlines.

Magnetic properties of the ground or objects in it.

Fluxes in the Earth's magnetic field.

Changes in saturation level of the soil, due to some areas having better drainage (perhaps because of buried features a la crop marks)

There are many expainable things that could be being picked up. It's what GeoPhys does innit except that the 'puter draws the map for you and the technology gives you credibility in the eyes of sceptics :-)