In the late 90's, the company I was working for rented me an Astra/Focus sized car for me for weekend working. The hire car company delivered me a Mercedes S class because they'd run out of other cars! Only problem was when I got to the petrol station, I couldn't open the petrol cap. Every car I had driven until then either had a pull cable under the driver's seat or just an exposed petrol cap with a key. I pressed every button and looked under every seat for a petrol-flap release button, to no avail.
No information in the hand book at all about opening the petrol cap either.
Eventually I had to put my pride on hold and ask at the petrol station. Turns out that the petrol flap unlocked when the car was unlocked and you only had to pull the flap open! So advanced these Mercs.