Seatbelt warning

[Sonic!!!!!]

Hi
Could anybody tell me how the seatbelt warning system works on the facelift XJS.

I have a celebration model, and when the ignition is turned on the seatbelt warning siren goes off and the warning light comes up on the dashboard, but after around 10 seconds the dashboard warning light goes out and the warning siren stops, even though I have not my seatbelt on.

Thoughts?