Generally here in the states this is how it goes down:

First, a law enforcement unit would either have to have a tip or actually fly over a large region first before sending out other aircraft to try and properly identify the crop. Contrary to what some people think, marijuana does not give off a heat signature that can be detected by any type of aircraft that local law enforcement would be using. Perhaps the ATF task force or the military, but not local government and law enforcement.

The way large crops are discovered outside is due to the heat given off by water lines. Water carries a temperature warmer than the ground when it it routed through piping under the crop as with irrigation. What gives the grower away is the fact that the water and piping under the ground actually warms up the surface so that small lines or "grids" can be identified by infa red. Outdoor growers in remote regions also need power, and often times the underground generators or the exhaust venting from them give a grow away the same way the irrigation lines do.

Thermal imaging equipment cannot be used to see through a house, not only that but it would be too much trouble to do that for an indoor grower. The reason police do not have to have a warrant to arrest someone (if they are caught) is because most growers choose remote areas either where the owner is no where to be found or if the land has no owner. This takes away all associations to the grower and also enables the grower to be more stealth. In this case, law enforcement can fly over whatever they and and put you in jail for growing even though they didn't have a warrant in the first place.

I am involved with emergency medical dispatch and also search and rescue and law enforcement personnel on a constant basis, some of which have pilots I have personally talked with about methods for catching pot growers. In fact, many pilots get a sort of adrenaline rush off catching growers.