I cannot say I was in Ready mode rather than On, because I was unaware of this distinction before reading these helpful responses, especially from
@Theoburns. I know better now. I’m pretty sure I turned the car off and then back on again, dooming my 12V battery. With outside temps in the high 40s, my wife may well have been comfortable without being aware that the heater was off.
Although this was user error, I don’t feel particularly stupid. It’s unfathomable to me that a computer-driven vehicle that has the ability to charge its little battery from its big one would allow itself to get into such a catastrophic state rather than rescue itself.
[edit] I should add that she had the seat heater on, which would have drained the battery quickly while keeping her unaware that she wasn’t getting cabin heat. Assuming, that is, that the seat heaters work in On mode.
It's a very easy mistake to make so don't feel bad. When driving no one accidently leaves it in ON mode because if you do you won't be going anywhere, and anyone will realize "Oh yeah I have to press the brake pedal" (plus it tells you that on the screen).
But when turning on the car temporarily it's easy to get fooled into thinking it's running since the display fully powers up and looks similar to how it looks when running, even though there are some differences, like the aforementioned warning message as well as some warning icons like check engine, etc. and also the absence of READY on the display. However someone who is not looking directly at the display probably won't notice any of this, which is likely if you aren't going anywhere soon.
If you have your seatbelt on when you put the car into ON mode you will immediately see the warning message shown above. Which again is the scenario where you are getting ready to drive off. However get this, if you don't have your seatbelt on, which is likely if you got in your car and plan to sit for a while, then after powering into ON mode you first get a fasten seat belt warning for the first five seconds, only then followed by the message to press brake to power on. But by then you probably won't be looking at the display. They should change that as there is no reason to display a seatbelt message in ON mode since you won't be going anywhere, it should go straight to the press brake to power on message. If the driver does then press the brake and power on then it can give a seatbelt message.
And if you open the car door to turn on the car, which is another likely scenario, for example there is a passenger who will be remaining in the car like in your case and you realize after getting out of the car that you want to turn the car on for them, in this case after pressing the power button and putting it into ON mode you get a door open message for the first five seconds, which you will of course ignore since you obviously know the door is open, this is followed by five seconds of seatbelt warning (even though you aren't sitting in the seat, plus again this is ON mode so seatbelt is not needed). Only after those two messages appear does the message to press brake to power on appear. Of course by that time you are well on your way into the shopping mall.
By the way the reason that I always capitalize READY is not for effect, that is how it appears on the display and is also how they word it throughout the owners manual. And ON mode is capitalized in the owners manual also. This topic is confusing enough, so for consistency I just stick with how they show it in the manual.
Seat heaters, that's a good question, so I just ran out to my car and tried it. Yes the seat heaters work in ON mode but it seems to be at reduced power. I had it set for maximum, which I usually find to be too warm, but it just barely heated the seat. Presumably that is to lessen the drain on the 12 battery. Even in READY mode the seat heating is using 12V, my understanding is that there is no high voltage power coming into the passenger compartment. That's one reason why the resistance heater, which is high voltage, is located in the engine compartment.