It was a throwaway line in a Washington Post article, one of the many stories about government surveillance in the past few months.
By September 2004, a new NSA technique enabled the agency to find cellphones even when they were turned off. [Joint Special Operations Command] troops called this “The Find,” and it gave them thousands of new targets, including members of a burgeoning al-Qaeda-sponsored insurgency in Iraq, according to members of the unit."
Being able to track a mobile phone, while switched off? It was the first time we had read about the NSA having such a capability, and a revelation that has far-reaching implications. For most consumers, when they turn off their handsets, they have a reasonable expectation that the device is powered off, is not emitting or receiving a signal, and does not have any piece of the mobile phone still 'on'.