Wiring Problem

I was taking out the lamda plugs yesterday and noticed a wire that was tucked behind the headstock. I did think the wire was the one in which I had just plugged the O2 Eliminator plug into. However upon closer inspection it appear not to be the case and was in fact what looked to be a spare wire. I had a look at the connector on the end of the wire but it was still not clear as to what it should be plugged into. Is it that the wiring loom is shared with other models and this is something spare, or is it a case that the wire has become disconnected?
All help appreciated.

Colours?
The wiring diagram is pretty good you can download it from the how to section.
If you describe wiring telling us the colours helps.

If I assume yours is a Euro model and the connector is a Bosch style with two female contacts. The connector is for the fuel tank ventilation valve ( US models only )

Kuerbis