Just went into my garage to retrieve my keys from my 2016 Prius. Accidentally left them in the tray when I parked it about 5 hours ago. Something was buzzing/running in the back of the car. It wasn’t just the common occasional sounds that they make. It was constant. I got it to stop by starting the car briefly and then shutting it off. Nothing smelled hot. Could it be due to the keys being left in the car for an extended period? Should I be concerned and move it outside for the night?
That's the fuel vapor containment/capture system running a self-test. Has nothing to do with fob in the car. Totally normal.
The sound is the leak detection pump, and you'll hear it from around seven to around twenty-two minutes, depending on how long step C of the test happens to take.
Check your Owners Manual near the beginning (page 8 in the 2022 manual) under “For Your Information” for an explanation.