They are? That's weird! I didn't think that was possible to do anymore because of the screens that are in your gas line to prevent this and to prevent anyone putting a substance in your gas tank. I believe they are located just below the the line where most gas nozzles fit it to fill your car with gas. I could be wrong though.
put video cameras on walls all around your car turn them on when your about to sleep. ask your parents to stay up till 3 o'clock in midnight to see stay up even more late if you dont see anything. but dont be late for school though! :)