Small motor or pump running after car is shut down

2020 Prius, 50k miles. Gas mileage 52+, power is very good. No operating issues But - several times in the past few weeks I have come into the garage and heard something running from under the rear of the car. It is a small electric motor sound. If I turn on the car in the usual way the sound stops and stays off once the car is turned off. Two times I know that the noise was not present immediately after the car was turned off - but I observed the noise a couple hours later, and not sure when it started or how long it had been running. One time I let it run to see if it would turn off by itself. I checked it after 45 min and it was still going. I am thinking that it might be the hybrid cooling fan, fuel pump, ??? I was hoping for some ideas before I started taking things apart.