Can anyone think of any mechanical sensor gadget that I can put on top of the door lock above to detect if it gets opened? In my place, robbers open car doors during traffic and steal or hold up passengers. So I need to make sure the door lock detector would buzzer if it is open or defective because it is at the back seat right side which I can't see.
In its simplest form, a buzzer detector device you seek could be assembled using off-the-shelf components.
Consider a window alarm sensor switch. One part is a magnet, which is applied to the moving part of the lock mechanism. The other part is the reed switch, which has two electrical contacts.
Connect a wire to a battery, then to a buzzer, then through the alarm sensor switch. From the alarm sensor switch, return to the battery.
You would adjust the magnet and sensor switch to be close to each other when locked. This places the "normally closed" switch in the open position, saving the battery and silencing the buzzer.
When the lock is lifted, the magnet moves away, closing the switch, connecting the battery to the buzzer.
The above description is a quick off-the-top-of-my-head design. One could use more specialized components and have a much smaller assembly but the design concept is the same. I have in the past created a brake light switch for bicycle brakes. When the brake lever is not engaged, the magnet keeps the circuit open. The magnet and sensor for that project had 4 mm diameter components!