Mystery Plug in the engine bay...

When exploring the engine bay - the engine in my van is the 1.9DG - I found the following 'mystery plug'....

It was hooked up on the back of the alternator, with nothing plugged into it. A quick posting on the forum was rewarded with an explanation - it is a diagnostic plug, used by VW to determine that the engine is at 'Top Dead Centre' (TDC).

Not a problem and nothing to be concerned about, it was clipped back safely out of the way of the belt and forgotten about.