Again, the gauge doesn't show if it's full, the only way to know is to weigh it. Your scuba tank is full of compressed gas, this is full of a liquid that boils off to maintain pressure. As long as there's liquid in the cylinder, you're only reading the vapor pressure of CO2 which is highly temperature dependent. Just try lifting up the tank, ft it's still heavy you're fine.
To use that chart: 500 psig = 515 psia = 3551 kpa. Scroll down the second column (vapor pressure in kpa) and you'll see that's somewhere in the neighborhood of 1 deg C, which is 34 deg F. Are you sure your fridge is actually at 40? Are you sure the gauge is actually at 500? Either way leaking 20 pounds of CO2 out in one day would mean you have a pretty big leak, it should be noticeable.