Try it again with no disc, he may have a coil-edge false signal. It may read side to side, but be off center for out and back. Turning off disc might show it to be some close by junk. Or there could be multiple targets or maybe something buried on an angle. Nails often read differently long ways and across. So there's lots of reasons to explain it.
Toss down a test coin to practice on. Instead of scanning 4 directions while standing in one spot, swing side to side, then walk around the target a quarter-turn and swing side to side again. In other words, turn yourself to the other direction and swing side to side. This will eliminate any front to back swing errors.
Pump or bob the coil up and down over the target, it will only sound when directly over it.
I prefer a separate pinpointer for locating targets in the hole.
-Ed