2014 Prius - seatbelt warning never turns off

In the last week or so my 2014 prius has had a problem where the seatbelt warning (light and continuous beeping) comes on when the car starts after the first few beeps. Im sure I dont have to say this here, but it normally beeps a few times when the car turns on and stops. Then when youre moving a minimum speed, if a seatbelt isnt on it will beep until you put the seatbelt on. Now it just beeps continuously no matter what, parked, driving, all seatbelts on or off, doesnt matter.

Ive tried disabling the seatbelt using a OBD2 bluetooth connector and several apps on my phone but it makes no difference. Any idea whats causing this? Could it be a failing seat belt sensor? Or is it a warning for / indication of something else?