Thanks and LOL for the comments so far.
I'm not really hopeful to get an answer here, just some ideas to keep an eye out for or maybe dig into to check something.
So I'll find the LCM module at some point and give it a visual. At least knowing where it is and how to get to it will surely be useful.
I know how hard (impossible) debugging intermittent problems can be when they are not present. Hoping this is a one-time only event, but hope ain't ...
I don't think I have a parasitic drain. About 8 months ago, I was having an intermittent one, which was mysterious until I noticed the HVAC blower would sometimes be on with the key out. Clear symptom of the FSR, so I pulled 50A fuse F64 from the glovebox fuse box and have had no voltage problems since. I actually think I may have been having that FSR problem for a couple of years, but only found it when it got bad enough. But I'll keep an even closer eye on voltage.
I know I need to get the engine work done and get this thing back in service. Right now, it is taking up a spot in the driveway until I can find time to do it (with sufficient priority). As an example of things higher up on the list, I need to do 5x tax returns for myself and family members in the next few days. After that, my son's Alfa is parked next to it in the driveway with its own major engine issue, and I need to get that rolling first. At least the BMW can be driven onto the street and around the block to make space to work on my other cars or open a lane for the garage queen to get out. One day ...