im pretty sure they cannot see through things like brick or black shingles during the summer during the day, too much heat, they would easily block out the lower temperatures of the grow room. winter may be another story though, mostly they catch you by observing your exaust fans, to them it looks like a waterfall of heat pouring out if they use infrared (heat) cameras. most do not carry this but, if they use EMP (electro magnetic pulse) sensors then you are boned, they will see the amount of electro magnetic energy coming from your attic or wherever you grow, it will look like glowing blue circles coming from each light