what happens if the door is left ajar

Pretty sure the 12volt battery drains like in a normal car... The HV battery isn't effected unless the car is on, in which case it would run the engine to keep both batteries charged.

 

In a properly equipped car???
About 15 minutes elapse, and the voltage management circuit kicks in, and extinguishes your dome light...unless your ignition is specifically placed in the "accessory" position, in which case the dome light stays on until you turn it off, or place the ignition switch car in the "off" position.

In a Prius???
IIRC...eventually.....the 12v battery will flat-line, and the car won't start.

OK....
So it's not all that bad, really.
Unlike properly equipped cars, the Prius will not allow you to use the 12-V aux plug (formerly a cigarette lighter plug before the world became more "civilized") with the car in the "off" position, so there is little chance that you will leave a phone, tablet, or other useful device plugged into your car to drain the battery.
I don't know as much as I think I do about these things, but if you leave your dome light on overnight??? I don't think it will kill your battery.
Maybe over a weekend...maybe not.
12v batteries will withstand insults like these a few times.
Simple....
Recharge it, and then you're good to go.

If you leave your door ajar often enough to damage your battery????
Well.....
I'm sorry, but you deserve to have to buy a few batteries.

If you happened to walk into your garage this morning, and you saw the dome light on???
Don't worry.
If the car boots up (starts) then you're good to go.
Try not to do that too often.
Also...try not to leave your keyfob close to your car, or you won't have to worry about leaving the dome light on anymore (proximity sensor.)

Don't sweat the battery.
Like I said....they will withstand a few abuses.
Probably???....your car is fine!

Put the "Giveacrap" switch in the "don't" position, and start saving some more gas already.
Prius drivers worry waaaaaaaaaaaaaay too much.

 

Upon attempting to lock the car, the system will inform you with a beep that one of the doors isn't shut tight.

So if you always lock, you have nothing to worry about.

 

Oh yea... that's right I forgot about that feature, mostly because it always happened at annoying times when I didn't need to know, as in when inadvertently pushed the FOB doorlock button while lifting stuff up out of the car...

 

You can turn off the dome light and the "map lights", as well as the hatch area light (the switches are "three way", that is, on, door actuated, and off). But you still have the two "door lights" and the foot area LEDs, which draw almost nothing. Turn off the interior lights that you can, and replace the door lights with LEDs, and you can probably leave the door "ajar" for a few days. Leave the lights on and it will be "game over" for the 12 V battery if you leave a door open for too long. Probably take a few days though. It's a bigger battery in the GIII than was in the GII.

 

Eventually, the 12v battery will die and you'll have to walk home.