A dear friend, a dear single mom hard working no money to spare super nice friend, filled up her gas tank last week one night. Her Ford F-150 gas tank, so a big gas tank. Went to bed and in the morning got in to drive to work, and had an empty gas tank.
Her gas had been siphoned.
I asked her how much a locking gas tank cap would cost, "Around $50". So less than a tank of gas (how sad is that?!).
I suggest, that if yours doesn't lock, you get one that does.
Gas is only going to get more expensive, and people are going to get more desperate, or just plain underhanded and mean.