When you come across a mystery signal try moving you and the detector 90 degrees around the variable signal. I found it was a coin on edge, most times and if i hit from different angle it would reveal a constant beep-beep side to side. MOSTLY, as iron nails and aluminum do the different number stuff too. Depends if you are in all metal mode, sweep speed, etc.
try putting KNOWN items in ground at first to get an idea of what they say onscreen. Penny, nickle, quarter, nail and so on. helps learning curve. also checks depth detection of machine compared to how deep YOU bury them. Knowing how deep you bury a baby food lid can help eliminate a lot of dug trash. when you see what machine says it is. Then bury it standing up and see...