Ive been siphoned once a LONG time ago and it was before gas caps had to be 'properly sealed' or you failed inspection. So we were able to go to Walmart or something and buy some generic locking gas caps. You needed a key to open the gas tank. Noone could siphon from me anymore. But newer cars have something build into the gas caps to stop fumes from going into the environment and when you get inspected, they test that. If that seal is not working, you need to get a gas cap from your cars manufacturer/dealership. You can't even buy generic gas caps anymore! bummer right?? Im glad that my little gas door on my new car cannot be popped open from the outside, you need to push a button on the inside of the car to open it.
phew!