[dogma1138]

It wasn't transmitting a proper cell signal, it was transmitting something that they could detect.

I would assume that you would profile phones (of a certain make and model) and based on the return signal identify them. This was used in the early days of in places where there wasn't high cellphone density to begin with.

[cmdrfred]

I wonder how that passed the FCC.

[dogma1138]

I don't think this actually violates any FCC regulations given the right circumstances a can of coke can probably be induced to create enough backscatter to be trackable via RF.