Electrical Field Proximity Sensing
Electical Field Proximity Sensing (EFPS) is a system very similar to the active electrolocation seen in some fish. A small electrical field is generated between two antenna electrodes, and perturbations in the generated field are detected. An example system using this technology is The People Detector. This system is able to detect objects (namely people) through walls, much how fish can detect prey hiding under sand using passive electrolocation. It reports variable range, and can detect objects from up to 12 feet away. A prototype of the system is also able to detect objects within the surrounding 3D space. Unfortunately, there appears to be very little publicly facing information on this project.
Lateral-line inspired UAV detection: Part 6