Battery fan running for no reason?

Tonight I got back home and I heard my Prime I left in the garage making a whirring noise. The battery is at 44%, ambient temp about 30F. The noise was coming from the back and could not be heard inside the cabin or wasn’t any louder from the cargo area. But I could hear it distinctly from below the rear bumper.
With or without charging cable plugged in it would still make the noise. Opening and closing the car also made no change. And while I was investigating, the noise changed, stopped and then it started again.
What is this noise? A fan? And why is it running “randomly” with 44% SoC and while not charging? I don’t remember ever hearing this noise in the past.
I am used to hear noises at random times from Priuses, but this is really random and inexplicable.