Thanks for the excellent responses on this. I learned a lot, and your team is greatly appreciated!
Took the car in for an oil change and the latest recall reprogramming, and they looked at the CEL issue. They couldn't find any active codes to pull, so the running theory is that it was a loose gas cap issue. I'll be making use to double-double-double check that cap every time. You'd think that Subaru could have a "loose gas cap" light on the dash instead of a "OMG YOU CAR IS DYING RIGHT NOW AND YOU WILL TOO BECAUSE REASONS YOU CAN'T FATHOM!" or something like that.
They also tested the 6-month old battery, and it was good to go.
You all rock! Thanks again for the help.