Why does the car chime when the door is opened and the keys are not in the ignition on a 2000 Mazda MVP?

The chime may ring on a 2000 Mazda MVP if a door is open and the keys are not in the ignition because the car has a secondary alarm system that is letting the driver know that a door is ajar. If the door is opened while someone is sitting in the passenger seat it might go off before the person exits the vehicle.