I dont know if you guys remember seeing a rogue trader episode involving these "parking" companies. They caught one company out who were using the cars for general use and running up miles on them after they had collected them from the customer. This certain company also stated that the cars were stored in a controlled and secure area. They put a tracker on the cars and found they were just storing them in a completey open industrial estate which anyone could get access too.

Lesson to be learnt here..? Get a taxi!