As far as I have worked out there are NO occupancy sensors on the rear seats, just seat belt sensors (there is also no occupancy sensor on the driver's seat, but that is another matter). As such it sort of assumes there's someone there and the image of the two seats appears on the dash irrespective. I think the idea is that you don't have to crane your neck around to see if the rear seat passengers belt up or not! Whatever you do the symbols (no seat belt fitted or seat belt fitted) go out after about a minute. However, once fitted, if the belts are unplugged whilst the vehicle is in motion then the warning buzzer sounds. Presumably the idea behind this is to stop children "fiddling"/"playing" and removing their seat belts without the driver being aware of it.