The entry of a code on the keypad sounds a warning in the FD so that the pilots can confirm through the camera who is wanting to enter and then operate the switch.
The normal code causes a single warning sound and the emergency code causes a continuous warning sound. Both these sounds would be easily detectable on the CVR.
Entering the wrong code does not lock the door or time- out another attempt.
To bar entry the switch (sprung loaded to the central position)only needs to be moved to the lock position and the time- out will commence. It can be instantly over-ridden by placing the selector switch to the unlock position.