when I was digging a target that turned out to be foil wrapper in 8" of fill dirt. The Ah Had was when I realized that the reason the target seemed to be moving around was because as I was disturbing the soil around the target I was probably changing the character of the return signal. I think is more likely to happen with targets that are "multi-faceted" due to being crumpled, bent in multiple places or has an irregular reflection surface like foil usually does.