Thermal cameras operate by looking at objects in a different band of light called infrared, which is the way heat transmits. Ordinarily, you could see a person from behind a wall or other solid object by looking in the infrared, as their heat still passes through that object somewhat, like with the garbage bag (DO NOT TRY THAT AT HOME).
In the case of plexiglass and glass, however, infrared light is not able to pass through, resulting in a blank spot where Jacob’s head and hand used to be. You can even see it with Jacob’s glasses, as they are cooler than most of his face in the infrared.
We actually use this property of plexiglass here at AstroCamp for several solar-powered devices. The inside of our solar ovens are black so they absorb visible light, heating up, and have a plexiglass door covering them so that heat can’t escape. We also use this for a solar water heating system where visible light strikes a black metal tube, which heats up and conducts heat well, then that heat is trapped by a plexiglass covering. Our IR camera is a great source of learning, but it’s also just a lot of fun.