No idea how this happened, but discovered it last night on battery change... Can't see the 'chip' anywhere, so may have happened a while ago.
It may be related to the fact that etrex arrived off ebay, with leaking batteries (hence reduction to £35). On the last ride it turned off with a full battery (for a few mins before i realised), and this may have had something to do with the fact that the end battery contact had come loose.
I've put some contact adhesive on the battery terminal plate and case, but was wondering how I could best build up the corner of this battery chamber, to make sure water can't get in?