I have a 2019 RX350. Yesterday I could not sleep and went to garage to catch a smoke. I noticed a humming sound as if some kind of a motor was running. I tried to get as far under the car as I could and it seemed it was coming from the area of left rear under the car seems around the fuel tank. Did you ever find the solution. I dont know how long it was running but I opened the car door and it went away.
Few days ago my wife said that she heard the same thing, and she thought that the door was open, she opened and closed the door and it went away also.