Actually, people don't use lead paint anymore. Sorry, dude. 8)
Frosty, thermal imaging was ruled illegal without a proper warrant in the United States for residential buildings. I don't have the link to the article because I have to find it within another forum, but it was the Supreme Court that heard the case.
Lets say the police were using a helicopter to do thermal imaging at night (best time to check elevated heat levels), and they found a few houses that were lit up. Before this evidence can be used, they need to be able to correlate something more substantial. ex. searching through the houses trash for any evidence, checking with the electrical company to see the houses energy use per day.
God help you of there is a CI that knows about your grow-op. Mylar doesn't prevent thermal imaging by the way. Keep things simple if your growing. Don't use more than 1000W per bedroom in the house. Don't throw trash outside in your trash can that can at all be connected to anything illegal. Use carbon filters. Don't tell anyone.