Lol, that got me too for a second (with no manual in the car), the first time I drove a 370 and had to go an put some fuel in tank. Looked for the button, then thought to myself, lets see if there's a release on the flap. Much to the amusement of my co-pilot as we were heading off to do a rally.
I got caught like this in a mercedes I had - I spent 10minutes walking around the car in the petrol station until someone took pity on me and just pressed the fuel filler cover