It all began with a drug bust in Indiana. Police on the scene used several cell phones left at the scene to figure out who was involved and make the arrests. The Judges that initially presided over the case ruled that obtaining numbers from the phones was similar to illegally taking numbers out of an address book without a warrant.
Judge Posner, who subsequently reviewed the case in appeals court released these statements regarding the matter:
"This is a fair literal reading of the Robinson decision. But the Court did not reject the possibility of categorical limits to the rule laid down in it. Suppose the police stop a suspected drug dealer and find a diary, but a quick look reveals that it is a personal diary rather than a record of drug transactions, yet the officers keep on reading. A court might say that acquiring information known to be unrelated to the crime of which the person being arrested is suspected is an intrusion beyond the scope of Robinson’s rule."
So at first he finds reason to agree with the original ruling and provides reason not to justify the search, and that it would be, in fact, an invasion of privacy. But he continues:
"We need not consider what level of risk to personal safety or to the preservation of evidence would be necessary to justify a more extensive search of a cell phone without a warrant, especially when we factor in the burden on the police of having to traipse about with Faraday bags or mirror-copying technology and having to be instructed in the use of these methods for preventing remote wiping or rendering it ineffectual. We can certainly imagine justifications for a more extensive search. The arrested suspect might have prearranged with coconspirators to call them periodically and if they didn’t hear from him on schedule to take that as a warning that he had been seized, and to scatter. Or if conspirators buy prepaid SIM (subscriber identity module) cards, each of which assigns a different phone number to the cell phone in which the card is inserted, and replace the SIM card each day, a police officer who seizes one of the cell phones will have only a short interval within which to discover the phone numbers of the other conspirators."
Essentially, the judge conveys the need for urgency in the situation and justifies the speedy work of the officers involved. He gets to the core with this statement:
"The officer who doesn’t make a quick search of the cell phone won’t find other conspirators’ phone numbers that are still in use."
In any event, the officers were committing the slightest intrusion possible by simply obtaining the number connected to the device and no other information from it. This is what lead to the arrests, and so the decision to allow the evidence in the case was reversed.
It sounds like this could be a case-specific ruling, and it may have diminished implications for future cases. While the judges explanation sounds a bit confusing, there is a lot of common sense coming through in what he wrote. Law enforcement is hard enough without making every step of the process a fruitless chore.