helicopters/ police staff and fuel costs dont warrant the usage of helicopters just to look for weed growers and if they did they cant tell what is causing the heat image.How sure are you that they cant randomly search like that??
The thermal cameras show difference in temp on the surface they are scanning as different shades - they don't see through the wall. You could use a cutting torch inside a room and not be visible unless the temp in the whole room raises to much above surrounding rooms.
A window will show hotter than the exterior wall in the winter, and missing insulation or air leaks in the building envelope will show up.
If one room in your house was much warmer than the rest of the house the exterior walls might show hotter, but if the room is close to the temp of the rest of the house it would not show anything.
The problem comes from dumping large amounts of heat out of the house - that shows up well.