It would not be a bad idea to attach a maintenance charger to the 12 volt battery and leave that plugged in (or in the sun, if solar), and additionally leave a key with somebody who can start the car every few weeks and let it run until the engine shuts off (traction battery charging complete).
It was sitting there on, waiting for input and the A/C was on. That is the closest logical reason I have for this (so far) being a one off anomaly. For a few minutes, or maybe hours may not have been a big deal, but for 12.5 hours maybe it was.
I would hook it to a Battery Tender trickle charger. That will at least keep the 12V battery healthy, and you can't do anything about the traction battery that I know of.
Interesting. Now I wonder how this applies on the Gen2. With my gen2 engine running, I'm not sure the MFD shows the battery charging while in reverse -regardless of the SOC. Something to investigate once again.
I can confirm that if my Traction battery is at about 40% SoC or on purple bars, and I’m trying to reverse, the engine WILL come on and start charging the HV battery immediately. It will put the bars back in the blue before shutting off. iPhone 6s +