I have actually experienced the issue on a variety of celebrations primarily with rusty iron things and even the odd Clay Dome that magically vanishes when digging the target.
I hear lots of individuals discussing the Halo Result and how it influences on buried targets in the ground whether they are Ferrous/Non Ferrous and even clay domes. The following short article originates from Fisher Labs Detector Manufacture and will help resolve some of the misconceptions out there.
The Halo EffectDownload
I wish to acknowledge the article to [Dave Johnson, FRP-Fisher 2012]