if it is connected to anything then it will be the airbag system for the tensions in event of an accident - if you have the airbag light on then this could be why. that is something of a theory though, not something I am certain on.
if you have access to a laptop and appropriate cable you can check it out to see if it says anything, if not I wouldn't waste your money getting a dealer to check it out.
if the light is still on then it should be registering the fact the belt is not plugged in.
of course this does come with an important safety message: always wear your seatbelt when in a motor vehicle and remember it is a UK legal requirement to do so.