Newly-filled Propane Tanks empty in four days-not used much

My propane tanks are always filled at my Union 76 station here in Bandon, OR.
They have a huge tank and know what they are doing. I just got two small locks at Ace Hdwe to attach to the bands around my tanks. It won't keep someone from being aggressive to take your tanks, but I think it will stop someone who thinks they might get caught. They will move to the next victim. These are small tanks.