I acutally had the same problem with my '01 Ford Taurus.
It would just randomly go dead.
The one time it went dead I was on the interstate driving for 4-5hours(long trip) pulled off in a rest area and shut the car off. Went in and when I came back out the battery was dead. I had only been in there for like 5minutes.
Since I was 3+ hours away from home and had road side assistance I called a tow truck. They tried jumping it, but nothing happened. I had them actually two it to an Advance Auto since it was about 8pm at night. They tested the system and told me the alternator was shot. I didn't believe them and just bought a new battery. I drove the car home and have been driving it ever since without anymore problems.
The only thing I can figure is that there was loose connection on the battery or something and caused it not to charge/go dead.
Beats me.