In America, it is acceptable for police officers to conduct a warrantless search of a vehicle when the officers have probable cause to believe that contraband, fruits of a crime, or other evidence is in a vehicle. This is known as the "automobile exception" to the warrant requirement. Also, police may conduct a warrantless search of a vehicle when a recent occupant (e.g. driver) has been validly arrested only under two exceptions--(1) when the arrestee is unsecured and within reaching distance of the passenger compartment of the vehicle at the time of arrest; or (2) when it is reasonable to believe that evidence relevant to the crime of arrest may be found in the vehicle. This second exception is known as the "Thornton exception." The above exceptions were clarified in the 2009 United States Supreme Court decision of Arizona v. Gant. However, Washington does not necessarily adhere to these same exceptions mentioned above.
In a recent story, police officers stopped a Longview citizen's vehicle because it did not have mud flaps (classic pretext). Then the trooper claims that he "spotted" a clear plastic container holding what the officer thought was marijuana. You may read about this and another similar story here.
It is typically unclear whether a driver actually gives valid, voluntary, knowing, and intelligent consent for an officer to search the vehicle (as officers claim oftentimes), but one thing is for sure in Washington--absent valid consent, the rules permitting officers to search vehicles are strict.
Unlike on the federal level, the "automobile exception" to the warrant requirement is not recognized in Washington. That is, even if an officer has well founded probable cause to believe that contraband or other evidence of a crime is in a vehicle, the officer still cannot search the vehicle without a warrant. Because of this, the officer must conduct the search pursuant to another exception to the warrant requirement.
Oftentimes officers conduct a vehicle search pursuant to the "incident to arrest" exception. However, the car search incident to arrest is also very narrow in Washington as well due to the recent Washington State Supreme Court case State v. Snapp. In Snapp, the Washington State Supreme court reiterated the fact that the "automobile exception" to the warrant requirement is not recognized in Washington. More importantly, the court held that the Thornton exception mentioned above is inconsistent with Article I, Section 7 of the Washington State constitution. Therefore, in Washington, an officer may only search a vehicle incident to an arrest when the arrestee is unsecured and within reaching distance of the passenger compartment at the time of the search. Because of this new ruling--officers have a lot more difficultly conducting car searches. As a result, it appears that officers have reacted by exploiting other exceptions to the warrant requirement.
Consent is another exception to the warrant requirement. As mentioned above, in general an officer may search a vehicle if the driver or owner of the vehicle provides the officer with consent that is knowing, intelligent, and voluntary consent. Getting consent from an individual is not always easy when the driver knows that contraband or other substance is located in the vehicle. Therefore, officers may "pressure" an individual into thinking that they have an obligation to consent to the search. Officers may make the driver feel as if he or she has no choice but to allow an officer into the vehicle (e.g. by telling the individual that they have probable cause and will get a warrant if the individual does not grant consent). Alternatively, officers may conduct an "inventory search." If the vehicle is impounded the officers may take an inventory of the items in the vehicle in order to "protect" the arrested driver from theft or loss of items.
Ultimately, a driver need not give the officer consent to search. The officer may truly not have probable cause to search the vehicle which a warrant will require. Additionally, although unclear at this time, since an "inventory search" is designed to protect an arrestee from loss or theft of items in an impounded vehicle, it makes sense that a driver should be able to decline an "inventory search" due to impound. This issue, however, seems to have reached the appellate level.
Overall, if a vehicle search is unlawful, then the fruits of such an unlawful search may lead to the suppression of evidence. If your vehicle was subject to a car search that led to charges, it is important to have an attorney who will evaluate this issue and fight for suppression.