Whenever an Airbag warning light comes on, a fault code is stored in the Airbag control module (ACM) or Occupant Classification Module (OCM) for the passenger side. It may take a specialized scan tool to access this fault code.
Edit: Some auto parts stores offer free Engine, ABS and other fault code readings in order to get increased business. If such a store is in your area, they may have you take the scan tool out (you may have to leave your license with them as collateral) read the code and let us know what code you find. The scan tool usually stores the code and the counter guy will print you out the fault code and information on it. Call around.
Shops may have to write up a repair order first for insurance purposes and have you sign it if an employee is to touch your car. If there is a diagnostic fee or charge, it would later be applied to the repair bill charges.
2006 did have the strain gauge in the passenger seat for measuring the needed deployment force for the passenger airbag. It could be calibrated with 3 different seat weights (infant, 5th percentile and adult) to vary the force of passenger airbag deployment. It was just a calibration procedure, no parts are needed. Seat belt switches may also trigger an airbag warning light. It could be many different things. It may or may not be related to the battery disconnect during the t/belt service.
Without knowing this fault code, we are just guessing.