Good question. My understanding was that if the car doors were open, the fuel door should be too.
Have you tried having someone unlock the doors while you listen in the area of the fuel door to see if you hear any sound?
MB 1986 190D in my past
MB 1987 300E on the street
MB 1994 'Smoke Silver' E420 in my driveway
1999 Mazda Miata in the fun stable
1964 E-Type Jaguar Coupe- Sold
1970 E-Type Jaguar Coupe- Sold
1968 Corvair Monza Conv. with Turbo Transplant- Sold
1986 Merkur Xr4ti- abandoned
various mundane American autos
If I'd known then what I know now...
Hell, I'd probably still have done it anyways.