In reply to biscuit and others:
Thanks muchly - interesting vid and article.
I keep my kit in a big bag too, but however you keep it it's difficult to stop contact between metals.
I was in Thailand at Xmas (forgot about that somehow), but still, it's only these krabs that have the damage. Also, when I look closely, it's affected my blue (definitely anodised) krabs as well as the plain grey ones (not sure whether these are anodised). But only the Pro-wires; other DMM models (and other brands such as Wild Country etc etc) are not affected despite having had the same exposure (which includes Thailand and more, as I've had them longer).
I'll take that advice and send some pics to DMM. Thanks all!