When you press for the first time the doors just lock and the engine cant start. When you press for the second time you activate the alarm sensors for the cabin. In case that someone breaks the window the alarm is turned on.
It's the double locking feature which also prevents the doors from being opened via the inside door opening handles if some thieving oik tries to get in through the windows. You can disarm the internal sensors by pressing the button on the drivers door before arming the system. This is handy if you have a dog or child in the car if for instance you are buying fuel.
The black plastic fuel filler adaptor is for emergency use, if you are having to use a jerry can or similar to add fuel. It defeats the easyfuel system built into the neck of the fuel pipe, which normally only lets the correct sized fuel nozzle to be used when filling the tank. This feature avoids mis-fueling! There are anti theft devices built into the filler further down the neck. So, keep the adaptor in the car, you won't need to use it unless you run out of fuel!
2bannounced , I think yours is fine. no central lock on the fuel flap, just the easy fuel device. metal flap cannot be pushed down by hand but a fuel nozzle can do it. i have read elsewhere on the forum that there is 2 flaps one of which is further down to stop the local scum putting non derv juice in there. i dont understand this bit but if you have a look on here its there somewhere? sorry i cant be of more help, but regards your car i reckon everything is fine. BK