Yes, thanks for catching my mistake. I was thinking along the lines of accessories, etc. But the 12V must also power all the various electronics that are always 'on', if even in a sleep mode. I have no idea what the SKS draws, but it's probably one of the more significant drains on the system.
There is not only the drain from those things that any car would have, but other items unique to the Prius, such as the coolant pump (though that's a one-time thing when you shut down). If I go into my garage when my Prius is just sitting, it's not unusual to hear it make various whirring and clicking sounds. One of these I understand to be the brake actuator, which usually can be caused to kick in if you step on the brake pedal with the car off. The manual mentions a pump in the back that checks for leakage from the batteries. And there are any number of other various noises which I'm at a loss to explain.
So yes, that tiny 12V battery has a lot to do while the car sits idle.