I'm at a crossroads right now also, my 2012 acura wagon has been fairly reliable, but the miles are getting up there. A few winters back, it started killing the battery overnight and the feared christmas tree appeared.
I purchased a $500 code reader/scanner and a factory service manual. Then went on a ghost hunt. Got nowhere untill a neighbor said he saw the cars headlights on at night. WTF?
I stopped leaving the headlight switch on "auto" and the problem never happened again.
I still don't understand how the lights could turn on with out a key present but thats modern cars.