Are you sure your 12V battery is healthy? A Prius starts its engine with the high voltage battery, needing the 12V battery just for booting up the computers. Needless to say, computer boot-up doesn't require a perfectly healthy battery. It may be possible that the 12V battery is dying, causing various kinds of voltage spikes on the low-voltage 12V electrical system in the car.
You won't be measuring voltage spikes on a multimeter, unless the multimeter can detect and store a momentary maximum voltage. You would need at least a digital storage oscilloscope to do that.
8 years is about the typical lifetime for a 12V battery.
The car electrical systems usually tolerate some amount of voltage spikes, but if you let the situation go to the extreme, it is even possible that some expensive electronics in your car will be damaged!
Please now do a health check for the 12V battery, or even replace it entirely based on its old age.
Oh, and poor connectivity to the 12V battery may also cause the symptoms. Verify all of the wiring if the battery appears to be healthy.