Hi
if you are usually the only person in the car, but put a briefcase (or a heavy lunch box!) on the passenger seat, or throw things onto the back seat, it can cause a problem with the seat-weight sensor thinking there is someone sitting there with their belt off.
The solution is on aliexpress where they sell plug in seatbelt clips, which do the same as plugging the belt in properly, ie a clip goes into the seatbelt receiver and closes the 'seatbelt is fastened' microswitch. For the rears it's even sensible to leave the seatbelts always fastened (if the rear isn't used much and the rear seats aren't folded down much) as it is safer, preventing any heavy objects in the boot penetrating the cabin in a crash.