Last weekend some thieves attempted to steal/siphon gas out of three friends cars that were all parked together up in the mountains. All they did was mess up the gas cap doors and got no gas, but it will cost some money to fix. Probably a good $100 per gas door I would guess.
Apparently the same thing has been happening in Hermosa and all over the place in the USA as people freak out over gas prices. Having a locking door or gas cap will only cause the thieves to break it, it's almost better to just let them steal the gas.

But, if you have a nice bright blinking alarm light or can stick an alarm sticker near your gas cap that might help prevent the scumbags from stealing gas...and if you have a garage, use it!