Pump like noise rear drivers side

So I've heard this about four times now at least. Twice this week and twice sporadically before.
I poke my head into the garage to see that the doors are closed at night and hear a quiet pump sound.
Tonight I walked around my 2013 Prius v chasing it. (I previously knew it was from the Prius)..

The noise is coming from the rear drivers side of the vehicle. To stop the noise, simply turn on the Prius and then turn it off. I know it's not the accumulator pump for the brakes because that runs when you start the car and I'd say is almost twice as loud as the noise that I'm hearing.

I haven't pulled the service manual to see what's back there and the car doesn't make the noise when running (not without ICE running at least).
Has anyone heard this and did you track it down? I'm concerned that I'll come out one morning to a dead battery (which hasn't happened yet). I'm still on my battery from 2013 FWIW. It'll get replaced eventually but has been so good to me, it's almost sentimental at this point.

Previous work to chase this:
I initially thought this was the accumulator pump. I've since manually bled the brakes to replenish fluid and had apparently no impact.
It's happened this week after my wife drove the car and she has a key fob that's eating batteries frequently. I doubt there's correlation there but worth mentioning.

 

Well there you have it. I guess the next time I hear it, I'll wait longer before hunting and check back.