2008 dead battery

I have a 08 Eclipse spyder GT. Drove it appx 150 miles  went into a store for appx 3 hrs   came out and dead battery. That I can tell, no lights left on of any type (Headlight dome lights etc) and had to have it jumped to get it going?  What else could have possily been left on or cause that much of a drawdown in just a few hrs? I am stumped  any thoughts?
 
Thanks

Im guessing a bad ground on the alternator