They could notice the heat if they wanted to. A few things to think about though, in most, if not all states it is no longer legal to do random thermo searches, also if they did, technically they can tell by the heat that is being exhausted from the grow room weather the activity is normal.
Also, the tractability of the thermo use by LIO has to be considered. If you live in semi industrial, or business districts then LIO would pick up thermo detection all over the place, street lamps, bakeries, pizza shops and so on, and they are not about to examine all of those possibilities to sort them out and try to pen point some poor jo that has a 1K or even 2K grow IMO.
Adieu