Triggerfish

You don't need the telephone providers to track down a cellphone any more.

Triggerfish, also known as cell-site simulators or digital analyzers, are nothing new: the technology was used in the 1990s to hunt down renowned hacker Kevin Mitnick. By posing as a cell tower, triggerfish trick nearby cell phones into transmitting their serial numbers, phone numbers, and other data to law enforcement. Most previous descriptions of the technology, however, suggested that because of range limitations, triggerfish were only useful for zeroing in on a phone's precise location once cooperative cell providers had given a general location.

The ACLU got interested and under FOIA, found that:

As one of the documents intended to provide guidance for DOJ employees explains, triggerfish can be deployed "without the user knowing about it, and without involving the cell phone provider." That may be significant because the legal rulings requiring law enforcement to meet a high "probable cause" standard before acquiring cell location records have, thus far, pertained to requests for information from providers, pursuant to statutes such as the Communications Assistance for Law Enforcement Act (CALEA) and the Stored Communications Act

IOW, using a triggerfish device gets around pen-register safeguards.

Hmmmmmm......

HT: Schneier