Gas door opens in car wash?
Are you unsure about whether or not it opens? Do you want it to? Or would you like to prevent it?
Does the gas door lock when your doors are locked? If so, are the doors locked when this happens?
My 2011 MKZ door does not lock, and there is no cap, so I’m guessing his is like that. Mine hasn’t opened in a car wash, he may want to put some tape on it (or have the car wash guy do it, like they sometimes tape down rear wipers.