Check valve, fuel tank
WARNING: This page is about a different car, the 2003 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
Courtesy of SAAB-SCANIA OF AMERICA, INC.
The check valve is normally in the closed position and allows flow only in the direction towards the tank.
In this way, the valve prevents "back-spit" when the nozzle is shut-off.