This case involves applying a two-century-old constitutional provision to technology that was nearly unimaginable even twenty years ago, much less two hundred. The approach used in Chief Justice Roberts' unanimous opinion for the Court is to assess the degree of intrusion and balance that against government interests. I think we should also look at the Fourth Amendment itself and see if anything is closely analogous. The First Congress's handiwork, ratified by the requisite number of states, goes out of its way to protect "papers." The emails stored on a modern smart phone have the same informational content as the "papers" of 1791. It is the message, not the medium, that matters here, Marshall McLuhan notwithstanding.
The asserted justifications for searching the phone immediately without a warrant are weak. The police can seize the phone, remove the battery, and put it in the evidence locker. It's not going anywhere, and neither is the data, while they explain to a magistrate why they need to search it. I have no problem with the substantive Fourth Amendment holding in this case.
But the police were not, or should not have been, the ones on trial in the case of the People of the State of California v. David Riley. The questions to be decided in that case were whether Riley carried concealed and loaded weapons and whether he did so to benefit a criminal street gang. Should the contents of the phone, surely admissible if the police had gotten a warrant, be excluded from evidence on the gang question simply because they did not know they needed to get one?
The Supreme Court did not say, and it was not asked to say. But I think the question is important.