This article on wired.com (includes video demo) describes a system which uses a multiple radio transmitters and receivers around a house to detect the locations of people within the house from their effect on signals going through the house. In is present embryonic state the system can already figure out locations. As one of the comments on the article points out, more detail may be just around the corner, as may less obtrusive installations. If this sort of thing becomes a major concern, radio frequency blocking paint may become standard for anyone concerned with their personal domestic privacy.