Hybrid battery replacement 2002 Prius

If the auxilury battery was the original, it was due for a replacement anyway. They likely just cleared the codes as a wait & confirm measure. Then when the identical ones showed up again, it was a good case for warranty replacement.

However, that does beg the question of whether or not the SSC was ever done to prevent leaks. A sealant should have been applied to each cell to keep that very situation from ever happening.
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In this case, I was curious about SSC 40G

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I think that after seven years it was reasonable to replace the 12V battery, but agree with you that $392 is a very high price. The parts counter price should be ~$160 and you may have had to pay an additional $40 for a new battery bracket and associated hardware.

Further as you pointed out, the DTC that were logged really had nothing to do with the 12V battery. The service writer should have advised you that the plan was to clear the codes to see if they would recur.

If you owned the car since new, do you recall receiving a recall notice to have the battery modules sealed? The car may have a sticker on the driver's door jamb showing the SSC number.

 

What was the odometer reading on your 2002 when the traction battery was replaced?

I've attached a copy of the dealer notification regarding SSC 40G so that you can see what was involved. The customer notice letter provided a lesser level of detail.

The traction battery in my 2001 was replaced at ~61K miles under warranty when the car was 5 years old, due to DTC P3006. This code means that one or more of the 38 modules had a lower state of charge compared to the other modules. This is a different issue than the problem your battery faced, where electrolyte leakage resulted in DTC P3009, the high voltage leak code. The battery sealing work had previously been done to my battery.

We've already seen a reasonable minority of Classic owners having their batteries replaced. The lucky ones have benefited from the hybrid system warranty; while the unlucky ones have had to pay for the replacement themselves (or in some cases have been able to obtain and install a salvage battery.)

I would expect that Classic owners in areas with high summer temps would see a higher % of traction battery failures as time passes, due to high ambient temperatures promoting electrolyte loss.