Dead Auxiliary Battery after 170,000 Miles

Sure.

The hypothetical warning light would be powered by what? That's right, the low battery!
(I once had a computer program that printed an error message when the printer ran out of paper, by the time you got more paper, you printed pages of out of paper messages, same thing)

The HV battery has lethal levels of power, Toyota decided to start the car and do operational checks BEFORE enabling the High Voltage circuits. It seems like a safety item.

My advice would have been to replace it every 4 years, just like the 12 battery in every other car made. If you prefer not to pay Toyota prices:

Optima DS46B24R Optima battery direct fit replacement for Prius 2004 + & Plug-in Prius with Instructions would be my first choice, it contains proved advice.

Toyota Prius Starting Battery - Exide Edge 51JIS Flat Plate AGM is a safe battery (AGM and externally vented) that fits as well.
There are other choices in other countries, and other safe batteries that just do not fit as well here in the US.

 

Ditto Jimbo. Beyond that, use another dealer. $200 to $250 would be a good price for replacing the 12V battery.

 

Rather than wait for the 12 volt battery to "die", I'm inclined to hasten it's demise, just change it every 4~5 years. Beats getting stuck somewhere, or having to purchase a battery at the last minute. Ten years on the original battery is pretty lucky.

 

As soon as I got my used Prius the first maintenance thing I did was change the stock battery out to a Optima Yellow Top. I'd rather not need to worry about it down the road. Also at least they built in the diagnostics thing where you can check voltage levels.

 

If it's any comfort, a bad 12V battery would probably never strand you on the highway. Once the car's in ready mode the traction battery keeps the 12V system where it needs to be.
Several years ago I resolved to change out any 12V battery once it reached six years old on the principle that the extra cost of an unexpected battery issue along with unplanned replacement will exceed anything I save by eking out another year or two.

 

Excellent! The very best any 12V battery in my household has ever lasted was 9 years, on the spouse's car (non-hybrid), getting only half that distance.

While that would be nice, I'm withholding complaints until other cars provide the same warning.

To expand on Jimbo's answer, this is a safety issue, especially for first responders at crash scenes. The hybrid battery carries lethal voltage and current, which originally created widespread fear of harm among mechanics, emergency personnel, and hybrid / EV critics. Some of this fear was overblown, but part of it was justified.

By creating a system where shutting down the 12V system with common existing procedures also shut down and contained the lethal electricity inside the metal battery box, with no high voltage coming out of that box on any wire, Toyota helped gain market acceptance of this new technology.

It would be nice to abolish the 12V battery completely, leaving only the hybrid battery. But this will require new safety designs, new emergency procedures, and new and deeper training for emergency workers.

 

The 12V battery health is very easy to test yourself. Measuring the voltage with/without the headlights on is enough to tell when the battery is close enough to end of life. Of course you don't even need the voltmeter if you know the service menu trick, but in any case a cheap voltmeter is under $10, so either way is easy enough.

Personally if I was to wish for Toyota including any more sensors/indicators in the Prius I think I'd like an oil level (as opposed to pressure) warning. In any case, extra sensors and indicators add cost, so there's always a cost versus need debate, especially when we're talking about things that are slow changing and relatively easy to check manually anyway (like with a dipstick for example).