... "authorities" just having to aim your own device at you to unlock/access it without a warrant. ...
When FaceID was demoed by Craig Federighi,
his face had to be looking at the screen (he did not have to directly look straight at the screen) and his eyes had to be opened for FaceID to recognize his stored, FaceID mapping and open the iPhone X. (FaceID setup is locally stored, facial mapping by the TrueDepth camera which projects & analyzes 30,000 invisible dots to create a precise depth map of the owner's face.) ...In the demo Federighi looked away from the screen and FaceID did not activate; the iPhone X remained at the Lock Screen.
...presuming... there will be an initial PW required upon a (re)start ...
...Yes, FaceID requires an initial PW after restart, as we're used to seeing with TouchID. ...Federighi inadvertently got to demo this, as his first attempt at demoing FaceID was blocked by the PW entry screen. He cooly picked up the backup iPhone X and FaceID activated as he expected. [Here's a link to David Pogue's report about what actually happened - he verifies FaceID did not fail -
Pogue's Report ]
Additionally, Federighi showed that Apple tested FaceID using professionally made, facial masks, and FaceID's mapping is precise enough to discern the difference between a mask of the owner and the owner's stored, facial map.
I think it follows that in a situation where an iPhone X's owner does not want to open it for authorities, the owner could simply keep eyes closed. ...If there was forcible effort holding the owner's eyes open, I surmise facial recognition would be negated since the TrueDepth (front) camera's view would not recognize the facial contortions as well as the added, physical elements (in the view) holding the owner's eyes open as all these elements would not match the owner's stored, facial mapping.
It will be interesting when real world tests take place, especially with identical twins, as the tests will answer these questions about spoofing. But, I think these tests will likely verify FaceID is a technical step forward in security for opening the iPhone X.