How to detect if a nearby parked car has started engine?
This sounds like an XY problem.

Your problem is you need to prevent damage to your equipment.  Your rabbit-hole solution is to assume that you need to retract your equipment when the vehicle begins to drive.  You are myopically focused on detecting when the vehicle begins moving.

Instead, I propose you prevent the vehicle from moving.  An electric or hydraulic bollard is cheap ($2000-3000) and designed from both a mechanical and visual perspective that a car needs to stay put. 

Give the user a flashy screen, like a car wash.  Show them the status, gallons filled, estimated time remaining.  Retract the bollard after your robot's sensitive bits have tucked themselves away, then change the screen to a big friendly "you're done!".