[wildbill]

you may be right, but the FLIR manufacturers advertise they can spot heat sources within walls.

[Svartalfiar]

you may be right, but the FLIR manufacturers advertise they can spot heat sources within walls.

You can, because the heat source heats up the wall, and the FLIR can see the heat difference of the wall. A hot water pipe will show as a line running down the house. A body in a room is invisible. If you look at a water tower, you can actually see the water level because the air and water heat the walls at different temps, especially on a sunny day.

I'd post a picture of a house, but a quick Google search doesn't find much. Any color in IR images is false added, it's a black/white image. Color is added as after-processing.