2010 Ford Fusion fuel cap

There are two types of gas caps for this vehicle, the standard twist cap and the Easy Fuel Fill with no cap, there is no key for these caps, unless the original cap was replaced with an aftermarket type with a key. If this is the case, the lock is broken. Is this the original cap? If you have the owner's manual it will show you the type(s) of factory gas cap in the vehicle.

I have the capless fuel system. And when I lock the car the door opens. Is that normal or is it broken? It still closes completly. But the fuel door can be opened when locked.

OK, the fuel door in the body of the car should remain closed after it is shut by hand and should not open at any time again, Some models require you to push the door inward like a button, then it will pop open. Other models have a notch you can use to simply let you swing the fuel door open. What you are describing is not normal, I think.