When it comes to these types of searches, the first question is whether the person has a “legitimate expectation of privacy” in the thing to be searched. The second questions is under what circumstances can the police search. A person has a legitimate expectation of privacy if: (1) the person has shown that they want privacy, and (2) society has recognized that privacy. The first question is relatively easy to answer. This can be easily demonstrated by the driver not giving consent to the police to search the vehicle they are driving. The second aspect, that society has recognized that privacy, is a bit more challenging to answer. In this specific situation though, the United States Supreme Court has addressed this issue head-on.
In Byrd v. United States, the United States Supreme Court addressed this exact issue. In Byrd, a driver who was not on the rental contract, but was given permission from the renter to drive the vehicle, had his vehicle searched by the police. The Court ruled that as long as the driver is in lawful possession of the rented vehicle, then they have a legitimate expectation of privacy. The reasoning behind their ruling was that: “[O]ne who owns or lawfully possesses or controls property will in all likelihood have a legitimate expectation of privacy by virtue of the right to exclude.” Based upon the Byrd case, the police do need suspicion before they can search a rented vehicle that is driven by someone who was not authorized under the rental contract but received permission from the renter to drive the vehicle. In other words, the police need probable cause to believe that there will be evidence of a crime in the vehicle before they can lawfully search the vehicle.