2000 Lexus RX 300 battery drain

Electrical drain is one of the most dreaded problems a technician want to look into. I am sure that he has done all these but, just so that we cover all the basic.

1. How old is the battery? If it is more than 3 years old, fully charge the battery and have an auto parts store such as Checker O'Reilly test it using their battery tester. The tester is more accurate than a basic voltmeter. If the battery is low (right after it was charged), you may want to consider replacing the battery.
2. Check and make sure that both battery terminals are free of corrosion and have tight connection.
3. Once 1 and 2 completed, attach a digital voltmeter probes to the battery. With the key off the ignition, turn off all lights and close all doors. Record the voltage. You have to pull one fuse at a time. Record what fuse was pulled and the voltage with the fuse pulled out. Re-insert the fuse. Do this until all fuses are tested. For every voltage fluctuation when a fuse is pulled, you know there is a power drain. Now you need to concentrate on what circuit the said fuse is controlling to locate your power drain.

To add on to Mr. Jack here's the ammeter way for a battery drain test