Very interesting observation. I recently took to a park with little success, which surprised me, it was an old park that should have some good finds. Two hours later i had a 1c piece that was on the surface and not much else. Scratching my head i went back the following day, turned the discrimination down and no notching with the sens way up as high as i could get it (not easy in modern environment with emi) I dug maybe 250 targets, figuring id clean it out. I similarly found that there's only one way to be sure.....dig everything. I had a fair few upscaled targets and got hung up on a large piece of angular steel running vertically with the point pointed toward the surface But I had the best mixed bag haul for a while inc one ring and a couple of predecs. My shooting is now like my alluvial hunting, make sure its all dug so you know you have the lot.