If it happens only when you're in the car alone, this might be a clue that you're talking about the seat belt warning chime? Do you have items sitting on the passenger seat when you're by yourself?Where so the 5 chimes come from, they are very loud and only go off when i am in the can alone, very strange. Also, when the chimes go off, it turns off my hearing aids.
If you're talking about his hearing aids I would think he would be wearing them whether he was alone or had a passenger with him. On second thought with some passengers one might want not want to hear.Its the capacitance of the small devices triggering the seat sensor.