It smells like gas. Leaky gas filler cap.
I run into this problem on my 325 i . The Gas cap is always wet when the tank is full or more then half full. It smells like gas and it is visible in the filler neck also. There is a small amount of gas present almost all the time.
I have changed the gas cap with the genuine part. It didnt help.
Can this be emmision system related or a fuell pump?