Car making noise hours after being shut off.

About three days ago, while taking the dogs out, I go through the garage, which is not heated, and I notice the Prius is making a noise. It sounds like an electric motor running but I can not figure out where exactly the noise is coming from. It had been raining, and the car was pulled in the garage wet. About 5 minutes ago, I heard the same noise while getting the dogs in from outside. Both times I came back in the house, grabbed a FOB, and hopped in the car and put it in ready mode. I immediately shut it off, and the noise was gone. It reminds me of the sound that the car makes when you open the drivers door, which I think is the master cylinder pressurizing. It is about 7 degrees F here, but I am sure it is a little warmer in the garage. Any Idea what this may be? The car has about 33K miles on it. It gets driven everyday. Today, I got home about 4:30, and this just occurred at around 9:45. Whatever is causing the noise, I am sure will put a drain on the battery, as it definitely is an electrical motor of some sort running, which will take some power. I would hope that the power was coming from the 12 volt battery, and not the traction battery, as it usually disconnects a minute or so after the car is shut off. The car was definitely shut down when I drove it in, and every light was off on the dash when I got in to power it back up, except for the flashing lock light on the right side of the display. The wind chill here tonight is supposed to go to -25 degrees F. I am concerned about the car starting if it pulls too much power from the 12 volt battery.