Here's a link to "newTIS.info" listing of several topics concerning the fuel door and its operation:
https://www.newtis.info/tisv2/a/en/f...ch?q=fuel+flap
Fwiw, anytime my X1 is unlocked I can open the fuel door with just a light press on this door, even though the keys aren't in my pocket or inside the car. Possibly the water pressure from the car wash was strong enough to release the latch and allow the fuel door to open.