Discussion on answer by user16029: Do the police need probable cause or a search warrant to search a rental car being driven by someone who didnt rent the vehicle?

It's not illegal to be in breach of contract. It's a tort, not a crime.

Being in breach of the rental agreement isn't the same thing as illegally driving the car.

@MatthewRead this doesn't make it a criminal act however; its still a civil matter. Do you have statutory law or case precedent to provide references to your argument? Im open minded and can admit being wrong, but I need facts to base that on Sir.

The definition of theft almost invariably requires an intent to permanently deprive the owner of possession. This is clearly not the case - the unauthorised driver intends to return it to the authorised renter, with a finite time period before it is returned to its owner. Breach of contract by the renter, certainly, but it is not, by definition theft. @Ivan

No, it wouldn't, because theft has a definition, and there is more than just taking possession involved. For example: "Theft or stealing is the act of ... dishonestly and without claim of right, taking any property with intent to deprive any owner permanently of that property or of any interest in that property ..." and "... In this section, taking does not include obtaining ownership or possession of, or control over, any property with the consent of the person from whom it is obtained, whether or not consent is obtained by deception." So:

1. It's not taking and therefore not theft by definition. 2. There is no intent to permanently deprive the owner of that property (nor a likelihood of this occurring, as required in other sections) and therefore not theft by definition.

How is it that the police officers knew that the driver was not authorized when they pulled him over to do the search? They didn't, and couldn't have. They found that out along the way.

The person that was driving the car (borrower) had no contract with the car rental company and has not violated terms of anything. They borrowed a car from a friend, and might or might not have known it was a rental car, and might have been under the impression they were fully authorized to drive the car since they were given permission. The person who rented the car may have been in violation of their contract with the rental car company (I don't know if they allow undeclared drivers... most don't like it but some may allow it).