1. A passive-tracking system for passively tracking a living being, comprising: a visible-light sensor that is configured to generate a series of visible-light frames based at least in part on visible light that is reflected off an object; a sound transducer that is configured to generate a series of sound frames based at least in part on sound reflected off the object; a thermal sensor that is configured to generate a series of temperature frames based at least in part on thermal radiation emitted from the object; a time-of-flight sensor that is configured to generate a series of modulated-light frames based at least in part on modulated light reflected off the object; and a processor that is communicatively coupled to the visible-light sensor, the sound transducer, the thermal sensor, and the time-of-flight sensor, wherein the processor is configured to: receive the visible-light frames, the sound frames, the temperature frames, and the modulated-light frames; based at least in part on data of the visible-light frames and of the temperature frames, determine that the object is a living being and provide an X position and a Y position of the object; based at least in part on data of the sound frames and the modulated-light frames, provide a Z position of the object, wherein the X, Y, and Z positions combine to form a three-dimensional position of the object; and passively track the object over time by selectively updating the three-dimensional position of the object.