For the past year, the ACLU has been gathering information on local law enforcement agencies' use of cell phone location tracking. (We've written about what we've learned here, here, here, here, and here.) In addition to everything we've discovered about location tracking itself, we've also learned about a number of other techniques law enforcement and the telcos can use when they work together. Sometimes the information came to light because, as with this telecom data retention chart, the information on the other techniques was mingled with the information on cell phone location tracking. Sometimes it was because law enforcement agencies misunderstood our public records requests and sent us everything they had related to telephones.
For example, we received a Verizon form from Pittsfield, Mass. that law enforcement could use, among other things, to request a "Voicemail Pass Code Reset" in the case of "immediate danger of death or serious physical injury to a person." We do believe that there are certain emergency situations, for example to locate a missing person, where using cell phone information without a warrant is okay. Also, one of the biggest problems with cell phone location tracking is that it is completely surreptitious—unless location information is introduced in a criminal case, an individual rarely learns that he or she was tracked—but in the case of a changed voicemail password, it would become pretty obvious pretty quickly. Dude, I can't access my voicemail.