Why might the battery on a 2000 Buick LeSabre go dead overnight a few times a year?

I am having a similar problem with my 99 Regal LS. Sometimes my wife drives my car to work, a couple of times when she was ready to drive home the battery was completely dead! Needless to say she wasn't pleased! I had to come and remove the battery, take it to nearest auto parts store for a quick charge then reinstall the battery. At first I thought this might be caused by the door lights, However, It was odd I never had this problem when I drove the car. I will elaborate more on this point in a minute. I started doing some research and found some helpful info on another automotive forum message board. Key items I searched for on the other message board are "battery rundown protection" and "retained accessory power". I found someone with a 98 Pontiac that was having a dead battery problem. It turns out an amplfier in the stock stereo system was staying on causing a parasitic drain on the battery. This could be relevant to my situation because my wife listens to radio going to work and has a habit of not turning it off. She uses the auto cutoff feature that cuts it off when you open the car door. Let me know if any of this sounds relevant in your situation. It is always difficult to diagnose extremely intermittent problems.