No brakes!

Assuming the local shop used Techstream and had no codes, safest way would to have Toyota thoroughly inspect Brake System while giving them all the relevant information you have about the experience you had.
1.how is your 12v Battery condition? I had a similar brake system issue (just driving straight on the freeway, with light pressure on brakes) one time, and that changing a new 12v battery it never happened again, however i did have the same brake system issue pop up a year later taking a turn, which may have been the combination of a bad abs actuator/ low engine oil.
2. How is your engine oil level? I have seen posts where oil level between the low and high mark can still trigger issues with the engine on turns.
Of course safest way is still having toyota dealer/ shop that is experienced with prius check the brake system, however at a cost.

 

Intermittent problems are always hard for the dealer to diagnose. They would be willing to try to chase down what is causing your issues, but it will cost you. My guess would be that you have a problem with an electrical connection somewhere. If you do not see any evidence of damage to your wiring from rodents etc., my next recommendation would be to pull out each fuse and relay (especially those in the fuse-box under the hood), one at a time, and check for corrosion or other deposits on each of their prongs. On a couple of occasions, the dashboard on our Gen 2 lit up (I believe I may also have heard a long beep as you describe), and to follow up on one of the suggestions I saw in this forum, I started pulling the fuses and found one with significant deposits on it. I cleaned it up and have not had that kind of issue since (fingers crossed).

 

11.9V is too low for your 12V battery (cold). Maybe take your battery to local auto parts place and ask them to load-test , and to charge it up.

 

12.2V is still low for a cold measurement. A healthy charge from auto parts place wouldn't hurt.

 

I'm not sure about Snap On's ability to read Prius codes, I would guess it is not able to, given that none are being reported for your car. (It is hard to believe that a shop that services taxis in BC, where 95% of the fleet are Prii, does not use Techstream.) It is also my belief that Torque Pro cannot query the Skid Control ECU, which would be needed to get codes from the braking system. If you plan to keep a Prius for any length of time, and especially if you would like to save money and DIY for many repairs, I would recommend that you avail yourself of a mini VCI cable and Techstream (available through many vendors on Amazon). Since that may take awhile, in the meantime you would be best served by pulling blink codes from the Skid Control ECU using the method described here and reporting them back to us.

 

Not really, there are other scanners just as capable as long as the right amount of money is spent on it. A shop that services multi-brands will find the investment in a multi-brand tool better value when compared to buying several brand-specific scanners or computer/software combos, so I doubt their tool, is not capable, given as you said, "95% of the fleet are Prii."

 

If you ever have the problem again, check and note your engine oil level, and if your in a safe situation, note everything that happened on a paper or phone before you forget some details.

 

I'm a little sceptical of your latest test place. The 12.2 volt measurement you got before is very low. As mentioned up the page, get an electronic load-test done. Places like Canadian Tire have them (at least one in North Van when I was getting a battery for our son). They also have a Motomaster compatible AGM battery. Phone ahead, verify stock and that they can do load-test, clear up this variable. If for example the Cold Cranking Amps is below spec, coupled with the significantly low voltage reading, it's time.

FWIW, a good condition AGM battery for the Prius should be reading 12.7~12.8 volts and CCA well above spec (which is something like 325).

If you want to DIY test, something like Solar BA9 is a good entry level tester.

That's a LOT of kms btw, getting to where if it's not one thing it's another territory?