They could simply queue the devices. FIFO. What are the chances that someone will be the 51st person to rent a car will be the same person who first rented th

They could simply queue the devices. FIFO. What are the chances that someone will be the 51st person to rent a car will be the same person who first rented the car? And if it does happen, what's wrong with just re-pairing? Finally, add a clause in the rental agreement that the renter is personally responsible for unpairing the device. Otherwise, their phone will stay in the car's device list until it finally gets pushed out of the system at an indeterminate time, which depends on future rental volume.