We wish Fluke thermal imagers had Superman's x-ray vision, but they don't. Thermal imagers can only measure surface temperatures, and some surfaces are very problematic, due to emissivity and angle of viewing.
For example, thermal imagers can't "see" through glass (electrical on the other side of a glass window is out), and mid- to long-wave imagers can't "see" through Plexiglass or Lexan. They also can't "see" through electrical panel covers or through walls. If there's enough temperature differentiation behind a piece of drywall, because of wooden studs, (missing) insulation, hot pipes, or hot electrical connections or conduit, then enough heat may travel via "conduction" to the wall's surface to make the thermal image look like x-ray vision.
Thermal imagers can't see through fog or water, although they can measure the surface temperature of water. And, most thermal imagers can "see" "heat" through various types of thin plastic, such as plastic garbage and grocery bags, even though the human eye cannot.