'13 wouldn't start today

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.

Any ideas what might have happened? Dead/low 12v battery? What's the part that was buzzing?

Thanks!

 

Thank you all for your input so far.

Brake accumulator pump is what I believe was making the sound. Yes I do notice the sound normally, but it wasn't making the normal "clean" sound it makes, it was also quieter - likely from low voltage my assumption. I would think that it would time out though when vehicle isn't actually started and out into ready mode. Never noticed that before, I usually get in and drive it. Maybe the pump was just running the whole time and drained the battery? I have been telling myself the brakes have felt a little different lately - but then again these Prius brakes don't really feel comparable to any other vehicle I've ever driven.

I'd think Toyota would put a decent battery in this car from factory. My Cruiser had this factory Panasonic battery that lasted 13 years and 160k miles before it finally died during a week of over 100* days. Our old Corolla I think still had the original battery from 2001 and it had 225k on it.

I do not know what else to do, maybe I'll just mention it when I take it for service. I went out to the car this morning and it started right up after sitting for 12 hours.

 

Yes, I did not know the 12v battery was in back. I vaguely kinda knew it was back there, but confirmed it yesterday, lol. I've only been DDing our Prius in the last 6 months (it was my wife's DD) since our 3yr old wants to ride in our Land Cruiser exclusively (has DVD player).

It's not like I'm not mechanically inclined, I am. I work on all my other vehicles myself, the Prius is just "new tech" and part of me is opposed to learning more about it simply because it's a Prius, no offense to anyone here. I mean, I own one too and yesterday was my first real complaint when it inconvenienced me.

I think I will mention what happened to Toyota, just so they can maybe poke around, but I'm definitely not going to buy their battery. I saw a lot of people are suggesting a Yellow Optima - so perhaps I'll go for one of those Sears Diehard Platinum batteries, which is the same battery at a better price.

Not too concerned with the short trips killing batteries in this car - my commute is 30 miles each way. I'm familiar with the concept though, I had this problem in college, doing lots of short trips in my old k5 blazer diesel that had dual batteries.