When I disarm the panel, the keypad displays ENTER OLD CODE.
On these panels there is a prompt in the passcodes section of the program which is called passcode length.
This prompt is defaulted at 3. If passcodes are programmed to be longer than this, when disarming the panel, after the third digit is pressed the panel will disarm and then any further key presses appear as commands to the panel. In this case we had a four digit passcode that ended in 5 which is the command for passcode change.
To resolve the issue ensure that the Passcode Length prompt matches the length of passcodes programmed in the panel.