“Hot rocks”. I run into it in the foothills and anywhere with non local “float” rock. They’re more beepy the further up the metamorphic/igneous grade you get from my experience. I posit it’s because they differ measurably from the soil you’ve (or the machine) ground balanced to. They can suck...
Do we have a metal detecting ping list? I’m heading to the river again with you son to see what we can on this lovely weekend. Two weeks ago we dug up some WWII Browning M2 .50cal machine gun shell casings that had been manufactured in Des Moines. That was awesome.