Why is 12V Battery dead?

I was starting my 6 mo old/3,500 mile Prius when it refused--- all manner of lights came on, the transmission display blinked and then all went black. I assume it is a dead 12 v battery and it is now on a trickle charge and seems to be responding fine. However I am totally puzzled as to why--no lights were on (tho' the hatch wasn't secure so the little dash door open light was on). It has been 3 days since I last drove it, but I have left it for longer (SKS off) with no problem. The audio WASN'T turned off, but when the car is powered down, why would that drain the battery? Plus the car was parked next to my office, I would've heard the drone of medical Cds if it was still playing. Does everyone turn off their audio every time they power down?? The ONLY different thing is that it has been parked in FULL SUN these past days, (in Hawaii that means HOT)--could that have anything to do with the battery draining?? And now do I have to get the dealer involved??