thought I'll update this after speaking to the dealership (Bill Buckle in Brookvale/Sydney): they advised that this can happen when I have a bag or something on the passenger seat - apparently there are some kind of sensors in the seat that trigger the 'no seatbelt' warning beep, even if it's something as light as a bag. Not sure why this doesn't get triggered constantly, maybe I had just done the weekly shopping or something and the bag was heavier than normal. I'll keep an eye on this!
Oh yeah, thats happend to me before. The passenger seat has pressure sensors to determine whether to enable the passenger air bag. It also uses these sensors to monitor the passenger seatbelt so that the seatbelt chime isn't constantly going off.
When I was moving early last year, I had a bunch of stuff in the front seat and it was enough to trigger the passenger seatbelt warning, but not enough to activate the passenger air bag. Basically the car thought a kid was in the passenger seat without his seatbelt on, so I had to pull over and plug the seatbelt in or listen to it chime for the entire trip.
I have sense used the seatbelt trick to disable the seatbelt warnings after the initial seatbelt chimes on startup.