When your gas runs out, you need to pump it at gas station. While the gas staff is filling up, you might probably out of your car to do something such as: go and get snacks from the store, go to bathroom, get fresh air etc. I believe that you leave your car unlocked because someone may stays inside or you leave for short time.
For your short period careless out of the car, the thieves can easily steal your wallets, purses and your important personal belongings. Take a look at this video!
I suggest you to lock your car when you are away. Do not keep your expensive stuffs inside. Sometimes, even you lock your car, the thieves still knock the mirror down and take your belongings. Be careful!