In califonia and hawaii they used to fly helicopters around with thermal imaging looking for growhouses(thus searching random houses) in order to get a warrant, but I believe that was ruled an unlawful search.

As for the material used to line your roof, I believe its aluminized mylar, the same stuff used to make thermal survival blankets. They're also carried by some forest firefighters to cover up with if they got surrounded by fire.

However, the military did some testing with this stuff to see if it could hold in body heat enough to evade enemy thermal imaging, and they found that the mylar just heated up to near body temp and didn't do any good. Perhaps if you used it to line your growroom the same could happen. But in the case of a grow room the heat is being tranferred by radiation rather than conduction in the case of having it against your body, so that may make a difference.