https://www.theregister.co.uk/2019/10/17/pixel_4_face_unlock/

Pixel 4 owners can unlock their smartphones with their faces even if they have their eyes closed. That's not good.

Google’s Face Unlock feature uses machine-learning algorithms to recognize your face and open access to the device's apps and data. The biometric system is designed to ensure that only you can only unlock your own phone, and if it works even when your eyes are closed then, theoretically, someone else could access your phones by pointing the front facing camera at your face when you’re asleep, or, er, dead. In other words, your smartmobe can be unlocked by someone if they can point the gizmo at your face when you're completely oblivious - potentially when you're asleep or dead.