Gasoline (or any liquid) doesn't need to be exposed to direct sunlight to evaporate. The best way to cut down of lost gas to the heat, is to always park in the shade. If for example you park on a hot street (try walking barefoot on hot pavement) the rising heat can effect your gas levels, since the gas tank is only a few inches from the hot asphalt.
Then again, maybe one of your neighbors sneaks over every night and siphons gas from your tank...