Battery Dead after 8 days sitting the garage

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I have a 2010 manual 370z. The car has 3,000 miles. Original factory battery. I left the car in my garage while out of town, so it sat for 8 days without being started. When I returned the battery was so dead that the dashboard would not light. I verified that I didn't leave anything on (light headlights...etc). I took the battery out and headed to walmart to let them check if the battery was good or not...they came back and said the battery was fine just needed to be charged. Took the battery home put it back in car and the car started right up.

Anyone know what would be causing the battery to go dead? I find it hard to believe that you can't sit the car for 8 days without the battery going dead.