People don't have any expectation that the license plate on their car cannot be seen by law enforcement.

People do not expect their phones to be feeding their locations to cops and they do not expect things hidden on them unknowingly to exist. But it really stretches believability that people have no idea that cops check license plate. Or that cops communicate with each other about license plates to coordinate searches or that cops have databases of license plates. Heck there's a whole trope in media that you need to swap license plates to hide from cops.

Next you're going to be telling us we need a warrant for an Amber alert.