Have you ever hunted in one direction and then gone over a spot in another direction and found a coin you could not believe was missed the first time? Or in some cases the third or fourth comeback?.
I tend to run the threshold loud enough to hear changes but no louder. I think that the target for whatever reason, too slow too fast, not careful , sweep speed, whatever makes the threshold perk up or falter but does not get a good enough read to target id or sound. This becomes more obvious in large drawn out hunts where targets are few and far between. But if the threshold stumbles I focus in on that area and see if I can bring in a target response. I tent not to do this in the average hunt site that is small as I think I am getting all the targets easy but on the large sites where they are few and far between I will focus more on this threshold change. It works once in a while and makes me think I should be more careful in the small areas as well.
good luck
utahshovelhead