I find a bunch of those in the coastal mountains of N. California...especially in campgrounds!
First one I found, in a riverbank, rang "silver" on my BH Pioneer...I was SOOO excited!...then, when did more research, found that it was melted ?....lead, aluminum, etc. Don't know why they'd be around campsites/grounds, but I'm always finding these...different shapes but around the same size as yours...have also found a few of them have very sharp edges (aluminum?), but most are smooth...