Try your detector on a bobbypin and see if that is what you are experiencing.
Then play around with GC, FC, High Trash modes as well as manual NC settings like 1, 6, 11 to see any differences (on the bobbypin).
Use open screen, no discrimination. I think in some modes it will switch from a null to a visible/audible on lower left corner.
One thing that will make you wonder is switching between Beach mode and Coin mode in dry sand. When I do this (usually when I'm not finding much in Beach mode) I seem to find a coin pretty quickly. My mind must not be discriminating coins from trash signals plus I get false tones from higher sensitivity. Even in Coin mode though I probably should create a better mask pattern to weed out some junk. By this time though I have been hunting for hours and just give up on gold rings and look for dimes/quarters, which are easier to hear and see on the screen.
I am an engineer and always am curious what is under the coil and have wasted lots of time and energy finding iron rods, tent stakes, two feet deep coke cans, etc.
Some of these were nulls or targets that jumped around iron/copper areas and changed as I dug closer. Best to have some kind of minimum disc pattern along the bottom that rises on the right to mask out larger variety of iron. Also a little on top right to mask iron wrap around that sometimes happens.