Why is my auxilliary battery running down?

sounds like the battery is defective. have a load test run on it. The battery is 3 years old now so problems can and might start to develop.

 

Assuming that no lights are turned on, you need to get a new battery. Although 12.4V means that the battery is reasonably charged, this does not confirm that the battery has adequate capacity to power accessories when the car is not READY. Also the MFD display voltage check has some inaccuracy in the displayed voltage.

When using the radio while the car is not READY, the car should be in ACC-ON mode (where no warning lights are on), not in IG-ON mode (where the dashboard warning lights are on) because the latter mode places a very heavy drain on the 12V battery.

 

Did you buy this car new?
If the car previous to your one week storage event had a dead battery and was jumped that pretty much kills the battery.
If you kill the battery and let that dead battery sit for any length of time it will begin sulfating really fast and will never ever hold a charge again.
If a car battery goes good and dead just once thats usually it in my opinion, especially the little AGM's in our cars.

Whatever you do go out of your way to not jump start our car. Its extremely hazardous to the health of our car. I've read many many posters here who have destroyed there electrical systems on our car just doing a simple jump start or inadvertently reversing the jump polarity.

If you must jump our car do not run the engine on the donor car.

You need very little current to jump our battery. In fact one poster say's he used a 14 volt Dewalt drill battery to jump start his Prius with a dead battery. I do not doubt it.

If you don't drive the car alot you may need a battery tender.
This is not a Prius issue but a Hybrid issue. Many new cars have issues with unused batteries dying.
Please see the products listed in article of this link.
I especially like the Solar power Battery tender's:


Battery Tutorial



BTW, my car is almost 3 years old and the battery is perfect. I have
left the car for days at a time with no issue. But it has never gone dead. Not once. If it ever did I would not bother jumping it I would replace it.

99% of the time Prius's go dead is because the owners do not lock the cars on every sitiuation especially in the garage. If you have a cracked door or the hatch is not all the way down closed (most common) that leaves the overhead light on and drains the battery.
A simple push of the key fob and the car will not lock alerting you to the fact of a door or hatch open.

In the garage at night I beep the car and just before I step out the door I turn off the garage lights so I can see if I have left the overhead light on in the car. The car does not beep about that. Caught that light on by mistake like 10 times in the last 3 years
with the gargae light off trick. And the car has beeped me like 10 times so far with a cracked open hatch or door.

Lock the car every single time you get out of the car and most of your worries will disappear in owning this car.

If you don't you'll be buying alot of batteries......

 

I replaced mine in my '07 last summer. The vehicle began doing a lot of weird things and errors randomly occured. Ocassionally, the car wouldn't start. But, then a couple minutes later, it would. I replaced the battery with an Optima, and it works great.

 

Built-in 12V battery test procedure given here:
http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html