I’ve had my 2012 Volt honk at me unexpectedly when I undo my seatbelt intending to get out of the car (I sometimes undo the belt but leave it running to make notes on display and odometer readings), then shut it off, then open the door, then change my mind and shut the door and it honks... probably a fob-left-in-car reminder... so if you think it through, do you remember doing anything out of sequence at the time, e.g., perhaps you started to open the door as you were turning the car off??? Perhaps you did undo your seatbelt before turning it off on that particular day???