There's the original pinball machine tilt sensor - a pendulum
(contact 1) in a wire hoop (contact 2)

Mercury or mercury-alternative tilt switches, that might not
be sensitive enough if the car is lifted only slightly to get
in on a towing platform under the back wheels

The problem you'll have is that you may not be parked on
a horizontal surface. Whatever sensor you use has to take
its reference from whatever the orientation the car is in and
that you want it to stay in.

A self-levelling gimbal housing the sensor that locks in
place when the ignition goes off ?

Perhaps an accelerometer on the whole body or IR sensors
that measure the distance between the top of the tyre and
the mudguard, or something like that which detects changes
to the wheel heights as the suspension moves

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