Don't laugh, but I still dig those zinc signals. One of them turned out to be my Rolex. (See my post under the 'Best Finds' heading) I found it between a bunch of fresh holes dug by someone else-- they probably left it because it was a zinc signal. (I almost didn't dig it myself for the same reason!) Only way to be sure is to dig EVERY target.
And, YES, I do wish they would go back to the bronze pennies. Or maybe mix some iron with that zinc!