Wi-Fi mostly works at 2.4 and 5 gigahertz, with wavelengths of 12 and 6 centimetres, similar in size to parts of the human body. As we move, we disturb the signals a Wi-Fi unit sends and receives, and researchers can analyse these changes in signal strength and timing without looking at the data being carried.
Around 2012, engineers used ordinary Wi-Fi equipment to look through a 15 cm hollow wall and tell whether none, one, two or three people were moving in a closed room. Later systems could tell apart up to half a dozen people, identify individuals by how they move, and work out which keys someone is typing and what movements and sounds their mouth is making. Many of these systems use modified equipment at close range, but they could allow homes to respond to people or call for help when someone falls, and they also raise questions about privacy.
via Wi-fi is watching us | Great Moments in Science, ABC Radio National (archived)