I agree that this is consistent with the way U.S. law has worked - testimony (contents of the mind) cannot be compelled, but a legal search warrant is the "key" to physical/documentary evidence.
My question is whether the contents of a smartphone might enter the realm of "contents of the mind" rather than documentary evidence. Can it become a spouse-equivalent that cannot be compelled to testify against its spouse? Is there a doctor-patient or attorney-client privilege? Or will it remain the equivalent of a household servant/office employee, who can be compelled to speak?
It's a matter of the nature of the "relationship" a person has with their smart devices and how that relationship may become even more intimate as technology evolves. Voice and gestural control of the world around us is opening even deeper levels of "intimacy." 'Every breath you take, every move you make... I'll be watching you.'
The benefits we may obtain from these new technologies require ever-deepening levels of trust - we'll have to trust our self-driving vehicles with our lives. That simply cannot happen if we have reason to fear Siri and company. Isaac Asimov explored that question extensively and concluded that we would not embrace robots, though his reasons tended to be closer to xenophobia than rational distrust.
At the least, the comprehensive nature of the information recorded argues for limitations on the power of a search warrant - the warrant can't be a fishing expedition into every aspect of a person's life.
What of a legal, warrantless search (such as when you've been arrested while committing a crime)? The comprehensive nature of the recorded information would argue that there is a reasonable expectation that evidence of the crime is contained within, so it would be very difficult to argue that a search is illegitimate.
To what degree is a smart device akin to an automobile, that cannot be subjected to warrantless search without reasonable cause? Is the smart device a place/situation where we have a reasonable expectation of privacy (again, a "place" exempted from warrantless search)? Do Pass codes and biometric locks give us a reasonable expectation of privacy?
So much more information is collected, both intentionally and incidentally, than we could ever have imagined in the days of paper and pencil. It is far more organized (i.e. susceptible to efficient search) than conventional evidence. Essentially, is a smartphone a direct extension of our mind? Its ability to make us more effective (never forgetting a birthday, appointment, or password) entices us to share. Its nonjudgmental nature encourages us to confide.
These are all interesting questions, but what of the answers? It's clear that "forgetfulness" benefits both manufacturer/service provider and end user - classes of data that will automatically be purged (absent a legal order to the contrary, of course - I can imagine an entire body of law devoted to this - a temporary, automatic, police-ordered "stay," while actual disclosure awaits a valid warrant...).
What of the "wrong finger" tactic? It certainly can fall into the realm of tampering with/destroying evidence or refusing to comply with a legal warrant. Since police have a compelling reason to prevent destruction of evidence, I'd think they will not compel an on-the-spot unlock, outside of cases where an unlock may avert immediate public danger. If time is not of the essence, it's probably better to obtain a warrant and perform the unlock under controlled conditions - with legal counsel in attendance. That's a matter of good police procedure, rather than constitutional protections.
