Fuel smells can be the worst. A small amount can stay with you.
Place the car on a lift and view all the rubber hoses in the area around the tank exit,fuel pumps, accumulator etc. View also the return hoses.
Does it only smell with a full tank. Try keeping it at half for a while (after you can't find visual evidence - look for staining it will still smell long after dry).
If it makes a difference full, you can expose the tank sending unit (and seal) through the first aid kit (I think).
One other possibility before you pull the forward trunk panel. In the gas flap area there is a drain hole for rain water to exit. This pipe is metal at both ends but passes through the trunk with a rubber or plastic hose which if disconnected can pass overflow fumes and/or leakage right into the trunk.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician