Well my point is that "everything" is NOT off with the key---I'm not familiar with all the new computerized crap, but here's just one example

On a typical stereo, esp with a heavy outboard amplifier, --even on a "stocker" Some circuits are wired so that the main output transistors are across the buss all the time. This doesn't cause any problems until "a problem arises"

Your alternator is the same way. You have diodes that are "hot" all the time. They CAN and do go bad, and I have seen diodes that "leak."

Without painstakingly analyzing the entire circuit of your vehicle, I don't know--but unhooking the battery will tell you if it's in the vehicle or battery.