Since the car doesn't normally start with the 12V battery, you won't get a warning when the 12V battery starts to die like you would with a regular car (slow cranking, etc).Thanks for all the help. After charging the battery the car started right up and I drove about 100 miles. I am certain I did not leave anything on overnight so I'm still unsure as to why it happened at all. I will look into causes if it happens again tomorrow morning. The battery has given no symptoms of weakening. Thanks again.
If you haven't already, pop the top off of the 12V battery and check to see if one or more cells are dry. If so, top them off with distilled water and put it back on the charger.
It is important to do that on a monthly basis, especially if you live in a hot climate. It will extend the life of your battery significantly.