The reason is simple. When detecting chains, the machine only sees one link of the chain or more often, the clasp. The same is true with rings that are broken...they signal very weak, if at all. It's just the dynamics of the way most metal detectors function, as they are geared for coin-sized objects and other "solid" stuff. The exception with chains is when they're "balled up" in a little pile, which is why most of them are found. Stretch a chain out lengthwise and it takes a hell of a detector to get a signal off of it, sometimes even when waved directly over it. I have found several chains that I was about to just pass on due to crappy signal but for some reason pressed on and found a nice chain. So if it sounds even remotely like it might be a good signal, it pays to dig it.
