I had one of those signals today that disappeared. It was very strange, Before I pulled back the flap there was a decent signal. pull back the flap and it was weaker. Dig and the signal disappeared. I dug deep and the pinpointer would not pick up anything. Put the flap back and the signal reappeared. I was confused as heck and the target was not in the flap. I left it there and then it occurred to me to recheck the ground balance. I made slight adjustments went back and recovered a piece of automobile trim about a foot down.The signal did not disappear this time. So the lesson learned is to make sure that ground balance is set correctly.