I was rummaging around in the engine bay of my Pug diesel last night just checking it over visually, when I spotted a black cable on the right hand side of the car (passenger side when standing in front of it). The cable was thicker than a single wire and on the end is a clip on connector. I followed the cable which was almost tucked under the battery box. It then merged with some more cabling which in turn disappears into the front wing.

I could find no obvious place where it could reach and plug in to. The car doesn't appear to be suffering as a result so could it be where a diagnostic machine could be plugged into?