That’s not the purpose of my question, rather than increasing device security:
once you have set your device for fingerprint or face authentication, you should also set a PIN for emergency unlock.
this is also means that an attacker could also unlock the phone by using the PIN, that can be easily eavesdropped.
by logging the unlock type of the device, the legitimate owner can check if his device has been unlocked by a different method from the one he’s normally using.
but this can be done only if you can retrieve the unlock method that’s just been used.