Everything that has a temperature (above absolute zero) emits electromagnetic radiation in the infrared spectrum.
Humans are infrared emitters.
Reference: Black-body radiation > Human-body emission (Planck's law)
Figure 2: Thermal sensing/imaging with LWIR and pseudocolorization. From Wikipedia - by NASA/IPAC.
Figure 3: Thermal sensing/imaging with LWIR (8-14 μm) - Evaluation of "Human Thermal Fingerprint at Long Distance" by the military (From "Wired" archives - slide 13).
Use of blood vessels/capillaries located just under the skin of the face
Possibility to be performed in the dark and below cover (e.g. mask)
Infrared depth detection technology, which can be considered as infrared LiDAR, consists of an infrared emitter which projects a grid of infrared dots on a scene/face and an infrared sensor which calculates distance/depth. It was originally used for motion detection for the game console Kinnect Xbox by Microsoft/PrimeSense. Kinnect 2.0 could perform face recognition and heart rate measurement. In 2017, Apple incorporated infrared depth detection technology in its iPhone X camera with the purpose of advanced face recognition. It consists of 30.000 infrared dots directed towards the subject's face to create a 3D full animated mesh.
Nostrils have different temperatures on inhalation and exhalation. By monitoring their temperature one can determine the frequency of inhalation and exhalation and thereby the respiratory rate.
Figure 4: "The Spectrogram and Cadence Frequency of a Walking Human" pdf - p.20
Figure 5: Doppler RADAR-measured gait. From “Radar Technology For Acquiring Biological Signals” - G. Greneker (http://truth.charleshontsphd.com/JCAAWP/2006_No_2/2006_127-134.pdf)
From Defense Threat Reduction Agency (DTRA)'s facebook post
"Our project highlighted here by Armed with Science - DoD sounds like a TV crime drama but its actually being developed to help military dismantle #IED in the field. U.S. Department of Defense (DoD) #MakingTheWorldSafer