What is target masking? Target masking occurs when one or more metal objects hide a desirable item from being detecting or alter the metal detector id response as a result of being above, below, or adjacent to a coin, relic or piece of jewelry.
As a result, old coins or valuable pieces of jewelry are often missed by detectorists even when their coil did in fact pass over the desirable item. This happens more than one might think, especially in trashy locations. Even supposedly clean parks generally contain a massive amount of iron bits and junk metal that may not be detected due to discrimination or size but that can still affect identification.
Several small flakes of rusty iron from a disintegrating staple may be too small to register on the id, but they may cause the id of highly conductive coin to drop down into the mid-conductive range where pull-tabs and other junk items are typically found. When this happens, the coin is less likely to be dug as many detectorists rely heavily on discrimination features.
If those rust flakes were a larger piece of scrap iron, the ID may drop into the foil or even upper iron range where the signal from the trash item can overpower the coin signal. If iron is discriminated out, then the coin may not produce a response as the metal detector has analyzed the coin and surrounding debris as one piece of iron.
I recently experienced this last form of target masking while metal detecting a playground with the Garrett Ace 250. I was running the relic mode which accepts all metal items with the exception of the iron notch. In this mode, iron does not produce a sound.