I always heard of the wardens having magnets, I have had them go through everything in my duck boat, but I think when I starting duck hunting they just glanced at the boxes. I started duck hunting in the 90s so I think steel was well established and the old timers were no longer sneaking lead in.
I will say a friend of mine had a random pheasant load on him once, we got checked and he palmed it inside his glove the whole time we were being checked!