1. A passive-tracking system for passively tracking a human, comprising: a thermal sensor that is configured to receive thermal radiation emitted from an object in a monitoring area and generate a series of thermal frames based at least in part on the thermal radiation emitted from the object in the monitoring area; 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 thermal sensor and the time-of-flight sensor, wherein the processor is configured to: receive the thermal frames from the thermal sensor and the modulated-light frames from the time-of-flight sensor; based at least in part on the data of the thermal frames and the modulated-light frames, generate a novelty representation of the object; based at least in part on the data of the thermal frames and the modulated-light frames, determine whether the object is human; if the processor determines that the object is human, generate, based on the novelty representation, a three-dimensional position of the human object; passively track the object over time by, based on the novelty representation, selectively updating the three-dimensional position of the human object; based at least in part on data of the thermal frames, track one or more thermal reference frames; and based at least in part on data of the modulated-light frames, track one or more modulated-light reference frames.