they do use thermal scanning to collect evidence against a grower. however its evidence, its not how they get caught.

in NY they recently had a huge bust by the DEA. those guys got caught because one of them was away on vacation and his scubber stopped working and he was blasting his garden spell thru his apartment building.

someone that lived there called the cops about the constant MJ smell.

so in conclusion I wouldnt worry about the cops catching you via a thermal scan. chances are if they have a scan on your house you are already caught.