They're using a near infrared camera which can't see much beyond what your eye can see except for TV remote LEDs and security camera floodlights. It can't see temperature unless something is almost red hot.
should be able to warm up a mask with a blow dryer
You can detect a person's pulse by amplifying the right frequencies in a video, because the pressure of the heartbeat changes the color of the face almost simultaneously. You could probably fool that by giving the mask capillaries, but that would end up being expensive.