If the car is not actually stolen, the police may still take action if it is repossessed. In some states, it is illegal to repossess a car without a court order. If the car was repossessed without a court order, the police may charge the person who repossessed it with a crime. Additionally, the police may impound the car and hold it until the rightful owner can come and get it.
If you find a car that you believe has been repossessed, you should contact the police immediately. The police will be able to determine if the car is actually stolen and will take appropriate action.