gas cap is the problem. though gasketed, i find that tolerances often prevent a tight enough fit. this allows gasoline to leak out at the fuel filler pipe. once out, the rubber that surrounds the pipe never really furnishes a liquid tight seal, so that small amount of gas will now run down the fuel filler pipe.
as you have learned, that puts gas behind the rear seat. never in large amounts in my experience, but enough to stimulate the olfactory lobe[s] in the interior of the car.
i find this happens most often under hard straight-line deceleration or when performing other high g left turns with the fuel level above half full. i have bought new gas caps, but i find that none of them seal tight enough. perhaps a double gasket will work.
this is a problem that i have been trying to solve for years.
anyone got a fix, i am all ears.