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Batteries from the factory are prone to not have a long life, it just happens.
You will receive messages from On Star for a period of time, until they clear out of the system, even though work has been done and completed, just an annoying flaw in their system.
If you do have a draw, why do you think that it would not drain two batteries? How low did your tender say it was? Did you try starting at that level to see if it would?
Two batteries are usually set up when you have numerous accessories that are going to be a huge draw on the electrical system, not just for a daily driver.