How long do you usually leave your car? A day out in the sun shouldn't produce an observable drop in your fuel level. Unless you have a leak in your tank or your fuel line. Get it checked immediately!

Anyways, if your gas tank and cap is fairly secured and your fuel line is intact, one other location where you might want to look at is your carburator.

Then again maybe your fuel gauge is already old and worn out. Check your reading when your starting your car. Then compare it after driving around the block for a minute. There shouldn't be a big difference in the readings.

Hope this helps ... keep us posted p're