OK, I guess that I did over-react to your post, and I was a bit dismissive with my comment on the diagnostic value of a test light. You are also correct in your assessment that nearly all of my practical experience with auto electrics is decades old.

That being said, I suspect that most/all of what a modern car uses the battery for when it is turned off is to keep bias across semiconductors. My voltmeter test would discern between that and a true drain.

I have a '90 and a '91 brick, and two later-model BMWs in the garage, all of which could use a battery terminal cleaning. I'll do some tests this weekend. Perhaps I'll learn something.