i am certain that you will find this problem to be a gas cap that does not seal tightly.

hard braking, high g driving will cause gasoline to splash up the fuel filler pipe. the bad gas cap seal will allow some fuel to spill out. when it does, it leaks into the area behind the rear seat.

it is a nasty problem. i have a 560sec that i cannot seal. no matter how many new gas caps i have acquired. i am in the process of fabricating a new gasket in the hope that it will seal that gas gap to the filler pipe.

hope this helps.