I lived in a "bad" neighborhood when I was in my late teens for awhile and we constantly had gas stolen out of our cars. We tried just about everything with no luck. Finally, after the cops told us that they had more important things to watch for, we came up with our own plan.
Whoever it was kept getting one vehicle more than the others, (probably because it had a huge tank..lol) an old impala. Well, the gas tank on those are behind the rear plates kind of up under the bumper... so you reach in "blind" more or less top take the cap off. We took a hobby knife (small 1/4 inch blade) and taped it to the gas tank opening so just a bit of the blade was sticking over the end of it. Sure enough, the thief came back real soon after that and evidently just reached in to grab the cap off because there was a nice little trail of blood leading away (We didn't have enough of the blade sticking out to do major damage but I bet it sure hurt like a bitch! ) and the car still had a full tank. They never returned either.
Of course nowadays the results would probably end badly but this was 20+ years back
