Can a police officer search my car without my consent?

In general, police need a warrant to conduct a search of a home or car, but there are a lot of exceptions, especially in the context of a car.  The most common exception is where the police have "probable cause" to believe that evidence of a crime is contained in the car.  Probable cause is that amount of evidence which would lead a reasonably prudent person to a particular belief.  The simple fact that your car was parked in an employee section of a parking lot does not give the police probable cause to believe there was evidence of a breaking and entering inside of it, but if there were some contextual evidence that would lead a reasonable person to believe that there was evidence in the car, that could change the answer.