Horn started sounding and passenger headlight blinking while parked

There must be a fault with the security system. You may have to use your push button remote, the unlock door, etc. Push button Fob that is used to unlock the car, etc. As it is the only way to reset the security system on many cars. Even starting the car with the key will not reset the alarm. If you do not have the push button Fob.
Come back and I will try to locate a reset procedure.