Abstract: A passive-tracking system is described herein. The system can include a visible-light sensor, a sound transducer, a thermal sensor, a time-of-flight (ToF) sensor, and a processor. The processor can receive visible-light frames from the visible-light sensor, sound frames from the sound transducer, thermal frames from the thermal sensor, and modulated-light frames from the ToF sensor. The processor, based on data of the visible-light and temperature frames, can also determine that an object is a living being and can provide an X and Y position of the object. The processor, based on data of the sound and positioning frames, can determine a Z position of the object. The X, Y, and Z positions may combine to form a three-dimensional (3D) position of the object. The processor can also passively track the object over time by selectively updating the 3D position of the object.