It is correct that opening the passenger door does NOT make the warning buzzer sound.
Open driver door, buzzer sounds. This indicates that the driver door lock micro-switch is working, as the buzzer is sounding when switch operates.
Therefore fault is most likely with the passenger door lock micro-switch.
If the warning sound was on all the time, then i would suggest driver door lock mechanism.
If you look at the electrical drawings, you should be able to simulate the micro-switches at the connecting socket and isolate precisely if fault is with door lock mechanism.