Battery Woes

[JCrank]

My job sent me out of town for a week. When I got ready to fire up my truck Saturday morning, the battery was dead. I have a NAPA 31c battery which was purchased in October 2004. To the best of my knowledge everything on my truck was turned off before I left for work last Monday morning.
I went back to NAPA this morning as the battery claims "18 month free replacement" on it's label. NAPA gave me a hard time.... said the battery only had 4.17 volts and that it needed to be fully charged for them to test it. I told them that was the problem the battery not maintaining a charge. The NAPA staff then tried to tell me that there is something on my truck drawing down the battery. There was just no way the battery could have lost so much voltage in a week on it's own. They told me that sometimes the glove box light stays on even when the glove box is closed. I told them my truck doesn't have one. The ythe ntried the same excuse, this time with the under hood light, I told them I don't have one of those either. It proceeded on down the line with to the courtesy light in hte cab. I told them I knew that light had not stayed on because I had removed the bulb from it .....
What I would like to know is has anyone else had a problem with battery draw down when their truck has been parked for several days and what was the cause of it ? I truly beleive my battery is the problem, but I want to hear what you all have to say.
Thanks,
Jon