Ultra wideband (UWB) would seem to be a good candidate technology. Ultra wideband imaging radar can penetrate earth and buildings, and is discriminating enough to detect human heartbeats and respirations. If one can see humans through walls and other barriers and measure their heart rate and respirations, I strongly suspect that individuals could be identified and differentiated based upon changes in these variables with movement. For example, there should be fairly regular relationship between my movements within or outside my home and changes in my heart rate/respirations. Compared to my daughter, who is half my weight and in a much better physical condition, the increase in my heart rate associated with even moderate physical activity is likely to be quite different than hers. Once I can be differentiated and seen through walls, I can be continuously tracked anywhere. All that is left to assign is whether I’m a good guy or bad guy. UWB also has the advantage of being very difficult to detect and jam.

Here are a few links regarding UWB:

http://www.informit.com/articles/article.aspx?p=384461

http://teams.eas.muohio.edu/CCLIEM/radar/

http://www.navysbir.com/n07_2/n072-127.htm

this excerpt from the final link:
*UWB imaging systems/surveillance sensors can operate from 1.99-10.6 GHz. These frequencies are sufficiently high, with short enough wavelengths, and wide enough bandwidth, to provide high ranging resolution, sufficient to detect heartbeat and respiration. RF’s longer wavelengths than optical and IR provides the advantage of working in all weather conditions.*