Mine has this feature, I'm sure it's a standard UK car although it sat about for a year and half before registered.
Perhaps the tensioner has been replaced at some point?
It's not that much use really, I know when I am wearing my seat belt, it'd be more use if it told me that there was a passenger not wearing their belt - but to do that a system would be needed where either the belts have to be clipped in when the seat is empty or pressure sensors would be needed.