In the United States if a person is stopped they can NOT search your car without a warrant unless you consent, however if they suspect something they can. If anything is found they will have to prove that they had "just cause" without a warrant or consent or it will be thrown out. I worked in the Law Enforcement field for several years. Tried to make it as short, simple & to the point as I could. Hope it helps. The most important thing about this question is what country you are referring to. It depends upon the laws of the state, the reason the person was stopped and if the officer(s) had "just cause". The terms "just cause" or "suspicion of probability" can carry a broad legal definition. Therefore, the police can search a vehicle. For more information you can refer this link.