Under the U.S. Constitution, law enforcement officers are generally not allowed to conduct an illegal search and seizure of any area where you have a reasonable expectation of privacy without a warrant. However, under the automobile exception, there are several ways for an officer to search your vehicle without a warrant. Here are a few common exceptions to the warrant requirement.
Probable cause
If an Illinois officer has probable cause to search your vehicle, they may do so without a warrant. If an officer has a reasonable belief that a person was involved in a drug crime, and a reasonable belief that there is evidence of said crime in the vehicle to be searched, the officer may have the probable cause needed to search the vehicle without a warrant. Probable cause must be based on a totality of the circumstances (e.g., an odor of drugs coming from the vehicle), not just a feeling or a hunch.
Consent
If you consent to a search of your vehicle, an officer does not require a warrant to perform the search.
Plain view
If there are drugs on your seat, or otherwise, in plain view of the officer, the officer is allowed to seize that evidence and continue to search your vehicle. However, the officer may only search the areas that could possibly contain evidence of the crime.
If an officer searches your vehicle unlawfully, they have likely violated your Constitutional rights. Evidence of a drug crime that was obtained during an unlawful search and seizure of your vehicle will generally not be allowed into court.