The only "loaded gun" you can have in a car in Washington State is a handgun. Rifles and Shotguns can't be transported in a vehicle if loaded.
If you have a Concealed Pistol License (CPL) the pistol can be anywhere in the vehicle. In your lap, on your hip, glove box, or under the seat. As long as you "conceal" and lock your vehicle when you leave your OK to leave it in the vehicle although you might find it gone when you get back. Just ask the Chief of the Seattle PD:shock: