I own a 2002 SC430 and whenever I cover it for a few days ( with nothing left on) the battery drains down! Checked the battery and alternator - all are fine. I'm beginning to suspect that automatic light sensor on the dash, or gremlins! The problem only occurs when I put a car cover over the car but not when I use an interior windshield sunshield . Any clues?

Do you have the alarm on. Since not using my alarm not suffered the dead battery problem.

Easiest way .. start pulling fuses, beginning with the sensor circuit. Eventually you'll find the drain.
Btw, nice setup aquanuke, esp the rims. How about a few pics.

This is a common problem with cars left unused for several days. All of the various systems which run while car is idle will eventually run down the battery. SOme dealers will install a small battery booster on cars which sit on their lots for long periods to prevent this problem.