The police holds up the phone, pointing towards the suspect:
Detective: "Is this yours?"
_Suspect glances in the direction indicated, phone unlocks._
Detective: "Nevermind, I got it from here."
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At least TouchID required physical assault to get you to unlock the phone. FaceID on the other hand can be defeated with perfectly legal attention grabbing techniques.
This doesn't seem to be a valid argument when discussing the police in the United States.
Without a warrant: No information taken from your phone by the police is admissible.
With a warrant: A judge can compel you to unlock any device with a biometric lock, regardless of what sort of biometric lock we are discussing. Fingerprint, iris scan, or face, it simply does not matter.
Warrants aren’t always required. Also, lack of a warrant just means the direct evidence they gather won’t be admissible, but it might lead them to new evidence. Also, it’s not just the police one needs worry about.
Even if an illegal search of your phone led to new evidence, in the US that new evidence would fall under a legal doctrine known as "fruit of a poisonous tree".
Fruit of the poisonous tree is a legal metaphor in the United States used to describe evidence that is obtained illegally.
For example, if a police officer conducted an unconstitutional search of a home and obtained a key to a train station locker, and evidence of a crime came from the locker, that evidence would most likely be excluded under the fruit of the poisonous tree legal doctrine.
Border agents operate under a different set of rules and have been searching mobile devices without a warrant or even probable cause.
However the ACLU and EFF have filed a new lawsuit challenging this behavior, now that the Supreme Court has ruled that the police cannot conduct warrantless searches of cell phones inside the US.
> Without a warrant: No information taken from your phone by the police is admissible.
Doesn't (or can't) law enforcement also use parallel construction (based on information obtained without a warrant)? I believe the point about what's admissible is not as clear cut as you state in a single sentence.
Detective: "Is this yours?"
_Suspect glances in the direction indicated, phone unlocks._
Detective: "Nevermind, I got it from here."
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At least TouchID required physical assault to get you to unlock the phone. FaceID on the other hand can be defeated with perfectly legal attention grabbing techniques.