Will not start, dash lights on

2015 Prius C, under 40K miles. I got going with a quick jump to the 12 volt battery, but here are the anomalies:

-- Dash lights came on, but when I went to shift I got the "put in park to start" warning message. Carefully held brake pedal down in P and got the same error message on shifting to R or D.

-- All of the warning lights stayed on during this process, instead of shutting off after a couple seconds in normal circumstances. Fuel gauge did not fill out. Horn and dome lights seem weak.

-- Tried a few times, and at least a couple times the dash lights shut off when I shifted back to P (trying now to recall whether I got the "goodbye" message).

-- Jumped to the fuse box connections with no change (maybe didn't get good ground connection...). Jumped to the 12 volt battery and everything went back to normal, drove around and no further trouble. So far.

This last suggests a 12 volt battery problem (only a couple years old but car often sits for a week or more, about 10 days this time, without starting - something I am trying to avoid, especially in cold weather). But I always assumed that if there was just enough in the 12volt to wake up the hybrid system we would be good to go. This seems to suggest that the hybrid system can be "partially" woken up unless there is some other fault that I should be concerned about. Ideas?