Plants do not have a heat signature. Thats urban myths and has been debunked many times in the past.
I can tell you exactly how they spot plants from the sky. First off its practice and training. The plant gives of a emerald green look that is mostly different than other plants around. It also has a very distinct growing pattern.
The helicopters are out mostly looking for guerrilla growers, that means out in the woods normally near a source of water and recently cleared out patches like trees down etc etc.
They aren't normally looking at houses for weed growing but they will do passes. Also the Supreme Court (in the US) has upheld that its illegal to use FLIR to look into people property to see if they are committing a crime. Its a violation of your 4th amendment.
I get being paranoid so let me give you a piece of advice.... if they are going to stay outside the whole time then get some plastic. 6millimeter thick plastic is perfect but you can get 3 mil and fold it over you know what I'm saying.
String the plastic up over your area, make sure it can drain if it rains. Or you could buy a premade cheap greenhouse. You could prob get one for $100.
