Prius (Gen 3) :: 2013 Suddenly Wouldn't Start - Dead Or Low Battery?

My 2013 has been problem free until this afternoon when I went to leave work - it simply wouldn't start, maybe a dead/low battery? It's a 2013 Prius II and just hit 40k this week and now due for service, so maybe I'll have them look into it?

Today was very hot around 100*, around noon I went to get something from passenger side and decided to crack the windows. Instead of going to the other side I reached across the console and pushed start which gets it into whatever mode it's called. Pushed the brake with a hand while pushed button again to turn on and operate window controls. Then I pushed start again to turn car off, which turned off display as normal. When I went to start it 5 hours later I got nothing.

No dash light up, dim dome light, door open light was on with door open so I knew there was some measure of power, I think I saw a check engine light near the door open light lit up at some point in my attempts too. I popped hood open to see where the battery was at, lol, it's not in the engine bay. In my frustration I finally started noticing a "buzz" sound of some part in the engine bay. Tried to figure out what it was - all I know is it's mounted on the firewall in front of driver, about knee height, behind/below the brake and power steering reservoir. I was kinda able to get my hand far enough to confirm it was buzzing/moving/active by touching a finger to it. Was able to get it to turn off with sever pushes of the start button - maybe the car was cycling the different "on" modes without turning on?

So I decided to try and jump the car. Friend who's borrowing a car luckily had jumpers - the weirdest looking jumpers I've ever seen. They were thinner gauge and super flexible, they were also two separate strands instead of being connected like normal jumpers. Anyways, I got nothing out of it. Was about to give up when someone id called showed up - used their jumpers thinking maybe the weird ones were bad. Immediately got a spark when I put the negative on the strut bolt, confirming I had juice.

Prius started immediately as "normal" (no idea how a hybrid starts after a jump...). Stereo was working, headlights, dome lights, windows - basically like nothing happened. I did notice the battery indication was a little lower than normal, but still had just below 1/2 bars. If it were low I'd imagine it would be lower. It never gets above 3/4 bars for me unless I'm coasting down a mountain for a while.

Dead/low 12v battery? What's the part that was buzzing?