Half dead 12v battery?

My Prius confused me (and several others today). All is well (as far as I can tell), but I thought I'd post my symptoms just in case anyone else runs into the same problem.

Headlights worked, interior lights worked. (Possibly neither were at full strength?)
The car started beeping in an irritated fashion as soon as the smart key was detected inside the vehicle.
Pressing Start resulted in a christmas tree of death, minus a few indicators. (I got the red triangle, but not the hybrid failure icon).
The fuel gauge started flashing one pip (tank was near full, 8 gallons at least). Sometimes it would read full, but flashing.
All the lights on the shift indicator came on, including the little lines between all the letters - made for an odd display.
The speedometer blinked 0.
The car could not be turned off unless all the doors were closed (took us a minute to get it back off).
The computer never IPL'd, no logo, nothing on the MFD, not even a flicker.
Car would not shift, nor would the engine start (obvious given the above).

This happened after I left the car all day to have it detailed inside and out, and the windows tinted. Obviously there was plenty of opportunity for problems, but the symptoms were so odd that I wasn't immediately sure of the problem. The behavior of the gas gauge made me suspect that perhaps it had been left on, and run empty till it died. But the symptoms didn't quite match. Similarly, if it had not responded at all, I would have immediately suspected the 12v battery. In retrospect - the computer never initializing was a obvious indicator it was electrical, not a empty tank - as was the fact that the electronics were just behaving oddly as hell.

There was a jump box sitting right there, and after a quick trip to the manual to figure out which plastic cover the terminal was hiding under and a recommended ground. (Sure, put the obviously color coded red terminal under a plain back cover.. And then recommend a ground point no the other side of the car beyond the reach of the cables...) I made sure the car was off, we hooked it up, and it started first try, happy as a clam. Fuel gauge ran up to 8 bars (so it certainly wasn't gas). The computer had lost none of it's brains (mileage, tank miles, radio presets, etc. were all in tact). I've been driving it around since, and it's been stopping/restarting without incident.

I'm guessing the battery was drained half dead. Enough juice to initialize some of the electronics, but not enough for any of them to work right.

Now - even with all the things going on, dome lights on, doors open and closed all day, smart key likely kept within 10 feet of the car, etc... I'm quite surprised it managed to drain the 12v in under 8 hours. I knew it was a small battery - but that still does surprise me, and makes me a little bit nervous. I may want to invest in a small jump box just in case.

Anything anyone can think of that might have caused this beyond just open doors, etc? Any state you can leave the car in that will quickly take the 12v battery to it's grave?

Anyway - if anyone sees similar strange symptoms - it's probably the battery

-Ken