Methods and systems may include a system including a first passive motion sensor having a lateral field of view with a first edge and a second passive motion sensor having a lateral field of view with a second edge that is substantially parallel to the first edge. The first and second edges can define a virtual beam. The system may also include logic to receive signals from the first and second passive motion sensors and determine a gait velocity and level of activity based on the signals from the first and second passive motion sensors.