Interesting USB bug that left my battery dead.
 

I wanted to charge an external Lion battery pack while I drove, so I plugged it into my '18 BRZ USB port, where it began charging immediately. When I shut down the car, however, it then reversed its flow and started trying to charge my car. This introduced current confused the car, which then did not shut down my radio or Bluetooth when I opened my door.

The next morning, the car's battery was dead. I charged it and all was well. When I don't have the Lion battery plugged in, the car shuts down and acts normally.

So, the moral of the story is that current applied to the USB port at engine shutdown might inadvertently command some accessories to remain powered up, draining the car's battery, eventually.

Cheers!

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Unless there is a good reason to allow DC current flow back into the vehicle from the USB port it would seem reasonable that a diode might be a good addition to the circuit. It may not be that simple, however, depending upon how DC and data are combined in the physical circuitry.