Randomly, I get the "Passenger Side Airbag Off" message on the navi screen even though there is a 100+ lb person in the seat. Is there something to check or a way to fix it?

 

The side airbag is different from the main airbag. If the passenger is leaning towards the door, there is a sensor that will disable the side airbag.

 

This.

There's weight sensors in the side of the seatback I think - if you push hard on the bolster then the airbag switches off. I assume it's just to prevent injury if the airbag deploys and the passenger is not sitting properly in the seat.