Gen 1 Turtle mode-warm weather

03 Prius in FL, has been running fine. Until today, when just out of the driveway the Turtle Mode light came on and the Battery charge indicator on the LCD display indicates 100% charge. Usually it's between 20% and 80%. This showed 100%.
Techstream shows no codes that I could find. Traction battery voltages range from 15.08 to 15.77.
In Turtle Mode, there is no reverse as only the electric motors are available for reverse. So if the traction battery is unavailable, the car can only go forward.
It was not unusually warm today (mid 90s).

Any suggestions? Will put a charger on the 12v battery tonight, altho I think that's a long shot.

<edit> while writing and posting, I let the car sit in the shade of my driveway, running, and the turtle light went out by itself. Leads me to suspect that the traction battery got too warm. However, an over-temp traction battery should set some fault code, and I didn't see one with my TechStream. Is that the usual behavior?

 

Trombone, it WAS parked in the sun, but the rear windows were open an inch. Did NOT drive with the A/C on. See my edit above for the resolution. The experience does leave me a bit nervous about this battery set performing in warm weather.

<edit> I keep a thermometer in the car, and the internal cabin temperature was 105F. Cooled down to 90F pretty quickly as I drove with the windows open. I DID hear the traction battery fan on full while I was driving with the Turtle Mode light on (total drive excursion was about 6 miles).