This is based on what i have seen for UK law enforcement
i have actually just watched a documentary on how they spot cannabis growers in residential locations.
They do fly around in a chopper looking for heat signatures coming from buildings, only if you are in a "hot" area though, so there would need to be some sort of tipoff 1st. They compare it to other buildings in the area to see the difference.
Even if you house/room is well insulated there will always be some sort of heat discharge, usually they can see it around the windows, around the outside of the chimney and so on.
Light also, if you have an open grow area and you happen to have a hps going through the night people are going to notice. especially if it is a cop in a chopper looking for just that.
This will not sent the cops straight to your door just put you on a suspicious list for them to check another time, and gather more info on the premises.
They also look for condensation on the windows so carefully hehe.
Also smell is a big issue as well. not filtering the smell is one of the main things that will give you away, when you live with it you get used to the smell, but neighbors or visitors to your area will be able to smell it especially if they are not smokers.
And yes as above the most common way to get busted is people knowing you grow and opening there big mouth to impress other people because they know someone cool enough to grow.
Here are some pics,
1st: one you will see the building in the middle being slightly lighter, that makes it suspicious,
2nd pic: basically the same but on the roof near the chimney
3rd pic: you will notice the difference one chimney being blue the other red.
Sorry for re-post, computer messed up.
Hope this helps.
Happy smoking all and safe growing.