The passenger's seatbelt warning comes on with no one in the seat.
This is a failure of the seat occupant sensor. This is very common on your vehicle. The sensor fails and makes the computer think that there is someone in the seat. The sensor would need to be replaced as the entire seat bottom. If you are not planning on repairing this yourself, consider YourMechanic, as a certified mechanic can come to your home or office to diagnose the issue in person and recommend a repair.