How heavy is your laptop bag? According to my passenger airbag sensors my latptop bag is at least as heavy as a small child.
The way I understand the system is that if the passenger is below a certain combination of height and weight, then the airbag is turned off and the seatbelt light is not triggered, but if you are a little heavier and/or taller then the seatbelt light is triggered.
The point of this is apparently to prevent injuries to small or under-weight passengers by a deploying airbag.
Awful nice of my truck to want my laptop to be secured by a seatbelt but not injured by the airbag. 
I recently saw on a TV program how the inspiration for weight sensitive air bag sensors was some music chair that Penn and Teller use in one of their stage shows. And I thought, “WTF? How obvious is it to put a sensor in the seat?”
I’m pretty sure my laptop bag is as heavy as a medium sized child, especially when I put in my work and personal laptops, the associated cables and power supplies, and whatever other crap I’ve yet to convince myself that I don’t need but still carry around…
Mine definitely weighs as much as a small child. Laptop and dSLR and junk.
It trips the same sensor. It took me a few days to figure out why.