Well it happened again
Saturday was very humid and drizzly. I had the defrost on to reduce fogging. Sunday morning, dead battery. I'm 99.9% confident that it's related to the defrost and humidity now.
It's going to the dealer for other stuff. I guess I'm going to have to give it up for a long time for them to be able to reproduce and diagnose...or maybe I should never turn on the defrost again.