See that is I think a further level of detail that may not be implemented. It is realistic but might be too much for the game… I would imagine it would simply be some passenger parks the car and flies out. Another random passenger (chosen at random) from a different flight after an x hour gap just walks to the garage and leaves with the car.
As far as my previous comment goes I was saying they will definitely implement rental cars and parking in some capacity. The level of detail and timing is something we do not know