I accepted the Prius bribe to look after my friends' house & pet again for a few days. This time I'm learning more about the cold weather behavior of the car...
For example: it was -18C / 0F when I went to use it, and the engine fired almost immediately on power-up. So I shut if off, made sure it was in EV Mode before I turned it back on, but the ICE fired again.
So it burned dinosaurs along most of the subsequent ~28 km drive. (While using about 20% of the pack capacity.) I even stopped part way through the drive & switched it off & on again to no avail.
I don't think it was heater demand because I turned the temp & fan settings down and the defrost off, yet the ICE continued to run.
Possibly due to battery pack temperature? The car had been sitting for several days - a true cold soak. And the pack was full, so it didn't benefit from the heat of recent charging.
Coincidentally, a Chevy Bolt went by in the other direction while I was puzzling over the Prius ICE and I thought: they're sure not having this problem!!