Parasitic drain

[John ADAM]

On 2005 Lexus LS 430:

The battery dies just after 3-4 days of inactivity.  I make sure ALL lights are off.
Battery is new, alternator is also new. 
They tested battery anyway and charging system and load test came out good.
In my other Lexus, same age battery works well even after 2 weeks of sitting on driveway. 
Battery, terminals, and alternator belt etc. checked out good by an EXPERT mechanic. 
He thinks there is an electrical short somewhere and advised me to go to dealer which I am reluctant to do at $135/hour to diagnose and with no estimate on how long it will take (usual dealer trap) them to diagnose and fix.
Don't want to spend a lot on this old car which otherwise runs well and I intend to keep it.  But frequent dead battery issue is very annoying. 

Can members please help me understand some possible/common reasons for parasitic drain on 2005 Lexus LS 430 or in general.  I am sure there may be other who experienced similar issue in the past.

Thanks a lot in advance.

[RX400h]

I would take it to an Auto Electric shop. They solve tricky drain issues (and very quickly) that dealerships cannot. Here's an example of a shop that is close to my area of town:

Welcome to Pacific Auto Electric | Best Car Repair Shop | Poway

If you can find a place like this, It's likely they'll be reasonable about cost compared to a dealership.