It happened once yesterday and twice (so far) today... My daughter weighs 138# and both the dash warning light & overhead warning light come on to indicate that the passenger airbag is off. ??? -Erika
It won't help you, but mine goes on once and a while too, with nothing on the seat. The issue if I remember correctly is that the sensor is crap. On top of that, it is not easy to get to. I ignore it since the seat is rarely occupied, but you may want to have it looked at. Reaching under the seat and gently wiggling wires might not be a bad idea, but the sensor itself seems to be the cause most often.
Your passenger seat weight sensor is going bad. It's a common failure in the 1st gen 2005+ and expensive to replace (something like $600 at the dealer). Mine was replaced at about 30K miles, fortunately under warranty. I believe that when the sensor fails the airbag will be active but you'll have to live with the annoying warning light. If you know anybody with NCS Expert software, someone on NAM figured out how to disable the warning in the computer: http://www.northamericanmotoring.com/forums/stock-problems-issues/211367-passenger-seat-occupancy-sensor-srs-light-no-more.html