After unlocking my door with the key fob and opening it from the outside I get a fast door chime. It does not do this if the door was not locked, nor when exiting the car.

Could anyone tell me what is wrong and a possible fix.

Tested the basics and those parts work OK:
Key in ignition, lights off, door open - slow chime
Key in ignition, lights on, door open - slow chime
Key in ignition, lights on, door open or closed, key removed - fast chime
Key removed, lights off, door open - no chime

04 Mustang GT

Thanks