Any vehicle with an alarm system will naturally drain the 12-volt battery. Prius is worse, it has that and also SE/SS. That means if you forget to disable it the SE/SS, you chances of a dead 12-volt increase. The auto-on headlight sensors causes a minor constant drain too.
So the recommendation is to simply disconnect the 12-volt before you leave for longer than a month.
That 2-week recommendation is a bit of CYB left over from the Classic model which had a smaller battery and (like all batteries) could not hold a decent charge in the extreme cold for too long. So Toyota just kept it. After all, 12-volt lead-acid batteries are never the same after a deep-discharge. Just ask anyone that has ever left their lights on overnight.
Anywho, the nice perk about Prius is that the 12-volt is pretty much only used for powering the computer. Everything is else (including engine start-up) draws from the 206-volt battery-pack instead, which is automatically disconnected from the system every time you power-off the car. So the demand is much, much, much less then when a traditional vehicle tries to start its engine, relying on just a 12-volt battery to do it.
JOHN
http://john1701a.com