Geoff , spot on and great exampleHave also seen a significant piece of wiry specimen gold that produced a very weak signal on a 4500 although actually found with one. The finder said it was extremely shallow.
My own experience of non signals was with a bit of radiator coil. The types that there used to be in bar radiators. Going over some broken bits of the radiator wire coil easily seen sitting on the surface, I could not get a signal. Must have been something to do with the way the wire was wound into a coil.
Perhaps the same effect with the behavior of wiry gold or when gold is interconnected within a specimen that the reaction field can cancelled out or be vectored away from the metal detectors coil.