If I understand your post correctly, you just discovered something that many other folks are also unaware of, and this is a good opportunity to explain how an open hole in the ground can effect the inductance pattern of a metal detector. By dropping the target back into the hole you just dug without covering the target with the plug and packing it down tightly, this causes a
huge disruption to the search coil inductance pattern and makes it very difficult to detect the target at the bottom of the hole. Many people refer to this as disturbing the halo effect, but I have found that it is more of a case of disturbing the coil inductance pattern. If you would have covered the target with the plug or soil and NOT packed it down tightly, you would have had the same results as not covering the hole. BUT, if you had covered the hole and packed it tightly, then you would have seen completely different results. You would have been able to raise your coil much higher above the target while testing this target. This is the reason it is very important to continue to "dig" when a good signal disappears while digging. In most cases the target is still in the hole, it is just deeper than you may think it is. Many good targets are not recovered because of this reason. Give this test a shot..I think you are in for a pleasant surprise.

Thanks,

Troy