continous beep but no warning light

I have a problem in a used "newly bought" 2010 Prius which is used for a taxi, i really need to find out what it is. I did a search but found nothing.

All of sudden the car starts making a loud beep inside the car and it just won't stop until we shut the car off. Seems like an alarm of some sort but i have no warning or nothing special on the cluster. It is a loud continous beep, so most probably not the seat belt or a door.

Anyone has a clue? I will bring it to Toyota to get rapped bad but when we try to take an appointment it always is in a week. I can't use the taxi with that loud noise with the clients, no way, i am losing lots of money on this.

Anyone know where it is located so i can at least rip it off? Seem to be deep in the dash.

thanks for any pointer.

 

no the other key is not in, i beleive i may have found the proble, we have a tablet in place at all time and i installed a 12V-110V converter that i put hidden in the dash, i beleive the magnetic field may be causing the problem. I will try to relocate it and see if it solves my problem.