I see trhat now that I test it. The warning, with the 3 passengers in the rear icon, changes theh images when each belt is pulled out a few inches. Thenn I would remind the rest of the passnegers to 'buckle up'. Whereas the front has an occupancy sensor too to remind me or them to buckle up.
Thanks! Mystery solved. i was thinking therewas a hidden strain gauge under the floor.
End of question and thread.