I have done test like that with the Equinox and other detectors. Of all the detectors I have done that test, the Equinox did NOT impress me.
One time I dug a hole 8" deep in a field put a nickel in the bottom, then scanned over it, the Equinox had no idea it was there. It would hit the nickel on the ground but not down in the hole,there was no adjustment that I was able to make that made it hit the nickel.
So I see it has a weakness of hunting around iron, that is something most people don't want too admit.
Very common to have that happen, with many or most makes and models. Naturally it can be a different result based upon the detector used, the mode used, the search coils used
(size and type) and we can't forget the Ground Balance that was used.
I recall being asked to stop by a place in California back in '88 and assist a fellow with that problem. I worked for a detector company and was on the road and this fellow's father was a very avid detector dealer. He had built a brand new home, but the yard wasn't planted yet because he wanted to level out an area in the dirt and lay out a test bed before planting grass seed.
He used a silver Dime and did the very same thing you described: Dug a hole, measured the distance to the ground surface. Positioned the silver Dime in the hold laying flat-to-the-coil, then checked out his detector on the silver Dime. No response. When I got to his house that evening he showed me what he was having trouble with and the two White's models he was using that "didn't work."
It took me maybe thirty minutes of that visit to:
•..1.. Get a signal on the coin with his detector.
•..2.. Get a signal on the coin with my personal detector.
•..3.. Get a signal without a coin present as I educated him on Ground Balance.
I accomplished
1 by simply filling the hole with the dirt he had removed to bury the coin at about 5" to 6" and compressed the dirt to be consistent with the surrounding ground matrix.
I then did step
2 to show him it was findable by his brand as well as my different brand detector.
Then, to demonstrate what occurred when he had told me about how he checked a dug hole prior to 'planting' a coin and there were already targets in the dirt, I used my detector and swept over than open, hole without a coin in position. All I had to do for
3 to occur was misadjust my detector's Ground Balance so that it was too negative. Then it falsed as I swept over the hole, or 'void', just as it did when I lifted the search coil away from the ground.
He learned a bit about Ground Balance, and a little about how ground disruption can cause a detector to NOT appear to work properly.
This same thing can result, at times, when someone cuts a plug, removes the dirt, and fails to get a good response, or as good a response as originally, when the object is still in or near the bottom of the hole.
Monte