Every once in a while, the seat belt buzzing warning sound goes on forever even with the car off. The only way to get it to stop the buzz is to disconnect the battery (I have a kill switch on the battery, so no biggie) or to leave the seat belt connected. To me this would seem to indicate something in the belt mechanism that isn't releasing or such, but thought I'd see about getting some thoughts before getting into it.

 

I think the very first modification I made to my XJS was to disable the seatbelt buzzer. I never drive without a seatbelt, and I don't need the obnoxious reminder.

Under the seat, there are two wire connectors, one is for the seat heaters and lumbar support, and the other is the seat belt wires (just two wires). If you simply unplug the seat belt wires it will disable the buzzer. If you want to actually fix the fault, I will bet you likely have a worn wire that is grounding on the metal frame of the seat.