They found hot pockets in the house, which were used to obtain a search warrant and subsequently bust Kyllo.
Fortunately, a 5-4 Supreme Court decision ruled the scan an unlawful search under the Fourth Amendment, requiring a warrant the police did not obtain. Score one for privacy, but the government is about to have a far more controversial and dangerous tool at its disposal to monitor what's going on inside your home.
Unlike a thermal imager, this device is already in your home – and you put it there.
How It Works
WiFi is electromagnetic waves in the 2.4 and 5 GHz ranges. It's the same thing as the light you see, only it can penetrate walls due to its much longer wavelength. Just like light (and echolocation) these waves also reflect off various surfaces and, when reconstructed properly, can be used to create an image.
Development of this technology goes back at least as far as July 2005, where researchers claimed at an IEEE Symposium that they had created an ultra-wideband high-resolution short pulse imaging radar system operating around 10 GHz. The applications for which were explicitly for military and police use, providing them with "enhanced situation awareness."