Hello, my name is XXXXX XXXXX I will answer your question. When do you smell the fumes, is it after refueling?
What I would recomend first is that you make sure that all the plastic plugs are in the floor properly under the carpets, this can be verified by putting the car on a hoist or drive on rack and verify this. I would also lift up the flooring in the truck as well. The fuel tank is mounted basically under the rear seat so you may need to lift up the rear seat to check the plugs there. If the tank was leaking fumes, I would think that there would be a code set for a evap leak (P0455 or one of the P045# XXXXX since they all are fof evaporative emissions.) I would also make sure that there are no exhaust fumes getting into the truck of the vehicle thur any openings in the back of the vehicle.
About the only place that fuel can leak out that would not set a engine code would be either a injector o-ring or injector itself or a issue with a fuel line, it a fuel line or injector is leaking you should be able to see fuel dripping or pooling somewhere.
If you would like and can give me the complete VIN of the vehicle, I will be happy to run it thru Fords website and see if there is anything on this for you. Personally, I have not seen any issues on either the 500 or the Taurus (both are basically the same vehicle).