You are correct, though I didn't make myself clear.
If you're running 2400 watts of bulbs in a grow area, each 600 watt bulb is going to give off quite a bit of heat. Unless you're continuously cooling these lights directly (as many do), the heat from each lights will rise/radiate upwards and heat the ceiling/roof right above the lights in the grow area. The same thing could happen to a wall next to a light. Either way, depending on how close the lights are to adjacent surfaces, and what's behind/beyond them, the heat from the lights might create "hot spots" that are potentially detectable as abnormal IR signatures from outside. This could be true even if the room as a whole were vented to an average air temperature of 75F.
The point is that having a normal-temperature grow room is helpful in avoiding IR detection, but its not by itself sufficient. You also have to make sure that your setup isn't also generating "hot spots" that could appear suspicious to IR surveillance.
Now in practice, you may have insulated walls and/or an attic, and if there is insulation between the outside surfaces and the heat sources, you probably won't create any hot spots like I described above. But this may not be a given depending on where you're growing and how.
Those hot-spots, by the way, include externally vented heat. You may think that venting hot air through a chimney, drier vent, or air-conditioner would "fool" law enforcement, but they're not complete idiots. Any cop smart enough to operate an IR camera is probably also smart enough to wonder why you're running your furnace in July, your A/C in January, or why you're doing laundry at 3am. If they're scanning you as part of a random scan of say an entire neighborhood, this might slide. But if they're scanning you specifically because they have a tip off or other reason to look for a grow op, they they will be more attuned to specific heat leaks.
As a relevant side issue, like everything else, IR imaging technology has gotten much cheaper over the last few years. You can purchase a high quality handheld IR thermal imager for under $2000 nowadays. That's a lot of money, though in perspective it may not be very much as a one-time business expense for a serious grow operation. Its certainly less than the cost of a bust!
More important, you can often RENT these, and I think that may make a lot more sense. I saw a price quoted of 300 pounds for a week's rental in England:
http://www.bis.fm/thermalimaging/index.asp
You can rent them in the USA for $500 - 600 per week.
With one of these you could potentially do your own examination of the outside of your grow area, see if or where you are creating detectable IR signatures, and then make whatever changes are necessary.