As a general rule, police officers can (and many times often do) run the plate numbers of cars that have not committed any moving violations. And the officer has the legal right to act upon any information that he lawfully obtains. The fact is that when someone drives on a public road, they display their license plate at that time, so a police officer who observes the plate can run the number on it. Further, if the plate comes back as , for example, registration expired, then the officer then has probable cause to stop the car and cite the driver. That having been said, there has been potentailly differing case law on this matter, so to v=be certain of your rights in your specific jurisdiction, you can consult direclty with an attorney in your area.