I have a 2015 Prius and just recently when I open the door, the car beeps once. I thought it was because I needed a new battery for my key but that did not help as it still beeps once when I open the door.
welcome! make sure all the doors and hatch are closed properly check the 12v health try your second fob test the new fob battery these are all just guesses, check the manual for the one beep meaning