Another possibility is that the evaporative emissions (EVAP) system is running the purge function when the engine has been turned off. This is when the fuel tank vapours enter the engine where they can be efficiently burnt. During this event, the purge valve is open, drawing air into the charcoal canister of the EVAP system and then this is purged into the engine along with the fuel vapours by running the purge pump when the engine is off.
