Flat battery after going away.

[TonyHSD]

Good morning, 

I believe these issues experienced with Auris 12v Battery been normal and to be expected.
The reason why your batteries has died after some time without use is because they had been already at half charge prior to leaving the cars for days. On top of that after the holiday period you managed to start the car for 5 min then if you cloud just let it remain in ready mode for an hour would have probably saved the Battery but start and stop uses the most energy which was the last nail in the coffin. 
I do use my car a lot indeed and this really helps with maintaining 12v Battery  charge as long as the car remains in ready mode the longest possible. 
Before 2018 the car was used only 20-30 min a day , sometimes left untouched for a week and once up to a 4 weeks during summer holiday and no issues with 12v battery.
But here is the trick: 20-30 min a day even these been only 3-5 miles journeys. If you can’t use the car each time 30 min or more, if you don’t use regularly best to have a smart charger and charge every month or once every two months. Like any other cars , you drive no problems, you don’t drive then there are problems. 
The only battery I had changed is the remote key, every two years, and when the time is up the car politely reminded me. 

[AisinW]

If we leave the car for more than 1 week, it is better if we remove the D/C CUT on the fuse box inside the engine bay.  It will disable body control  unit including dom lights and remote system.  However, it is much easier than crawling around opening the trunk to re-attached the negative cable.  Make sure if we remove negative cable, do it completely. Not just loosen it up and slowly unplug it. Sometimes it can trigger C1241 code and can cause MOT failure.  It is my personal experience.  So, loosen the bolt and plug it out right off the Battery. IF we slightly connect the terminal for few second from loose connection, it can cause the ECU read below 10V and trigger all kinds of false codes. 

[Roy124]

Not strictly comparable, but my Yaris Cross with a resting Battery voltage of 12.46v drops to 12.04v as soon as the car is started to go to Ready Mode.  It then immediately jumps to over 14v.

I have not looked at the sequence if you go to ready mode and then switch off after a moment.  

[TonyHSD]

15 hours ago, Roy124 said:

Not strictly comparable, but my Yaris Cross with a resting battery voltage of 12.46v drops to 12.04v as soon as the car is started to go to Ready Mode.  It then immediately jumps to over 14v.

I have not looked at the sequence if you go to ready mode and then switch off after a moment.  

That’s normal. Last time I was playing with Carista and car was set in acc mode for 5 min or more and the voltage dropped to 10.5v , I was err, new Battery, but nope, pressed the brake push the button and the car started as usual. Then showed 14.7v charging and at some point 15v as seen by the ecu displayed on the Carista phone screen. 

[AisinW]

Battery drain pretty fast in  ACC mode.  I am not sure how much but it could be more than 10Amps.  That's why even a new Battery will always below 12V in ACC mode.  Typically, it is about 11.7-12V, never more than 12.3V.   During start up or shift to Ready mode, it takes 60 Amps, and very often the Battery voltage drops below 11V.  As long as it never drop below 10V, the car will not have an issue or give a DTC/check engine lights.