Not to be confused with a single beep when turning off a Prius, you might also encounter a single beep when opening the driver’s door on a third generation Prius (either to get in or get out of the car). For several days and pouring through the owner’s manual I’ve been trying to work out what was causing this mystery beep.
Well, I finally worked it out. This single beep happens when the sunroof is not completely closed.
Our sunroof was maybe 99% closed so from inside and outside the cabin it looked like it was shut but pushing the close toggle did result in the familiar noise of the sunroof closing up properly. After that, the beep was no more.