Point being a detector doesn't know one metal from another. It's just a device that measures relative resistivity and conductivity in relation to the surrounding soil with parameters set to favor flat & round coin shaped objects. EXCEPT iron, ferric minerals, cause a negative phase shift while other metals - lead, gold, silver, aluminum, cause a positive phase shift. A clad quarter of 93% copper 8% nickel looks JUST like a 90% silver 10% copper "silver" quarter to a detector.
Better detectors will have better programming and decision making circuits, but they still don't know silver from clad when it comes to dimes and quarters.