All the radio signal being broadcasted around us, then there is more or less a constant rainfall of signals being bombarded every where at all times.
Now insert a human, with his own electromagnetic field, and it would be like holding a hand under that waterfall, the signal would be weaker where the human is.
Now take the fact that they could potentially take the Raw data, and make a time laps of anyone moving through an area, using the signal strengths of a local area at that particular time.
Now to be honest, then this idea was presented in the show called "Continuum" however, i see it as very plausible (i may be wrong).
Now i know this technology will (or may even exist now) at some point exist, the question is, are the Sci Fi writers of Continuum just using today's technology and imagining what happens when it goes amok, or is it predictive programming... hmhmhm
Just my 2 cents
edit on 21/8/12 by WiseThinker because: (no reason given)