Inventory your guns and note which ones do not have a paper trail back to you.
Any bought from friends or before the 1968 Gun Control Act became law should be sequestered in a save place as a junk yard or an area where there is enough old iron in the soil to mess up their detecting equipment.
A place that has lots of old nails from old buildings torn down and burned years ago would be a good place.
With private sales still allowed in many states, just print up a bunch of bills of sale with names from the obits several tows away.
Worn out brass will make detectors sound off, too.