It's true, they are looking for escaping heat radiating from your house or a window, it is also possible for them to see blazing hot HID bulbs through windows and some thin walls. Normally some rooms are obviously warmer than others in any house but a full on grow operation with HID's will exhaust air that is sometimes 90 - 100 F temp wise if cooling the bulbs and venting that very hot air.

So, if you do have to vent air into your attic make sure it's broken down into a diffuser box and centralize it as close to the middle of the roof as possible not right by the edge or vent outside. You can even run it back into the AC box to mix with cold air and blow back into the house out of the growroom and that in turn will just make your AC run a little bit more because the house will then be warmer. (but no heat going outside) Obviously you would want to make 100% sure this air is odor free and filtered through a activated carbon filter in the room first before it comes out and goes anywhere the smell could be detected. Carbon filters really work, they are worth their weight in gold. If you are not running exhaust through a carbon filter you need to get one ASAP. You can eliminate a bunch of these potential pit-falls with high powered LED's and an IR blocking grow tent in the middle of a room.