flir doesn't look for the heat in walls of houses.... because of that very reason, houses have insulation.
they look for the hot exhaust which lights up the FLIR... then the cop sitting by the FLIR looks with his bear eyes to see if the heat is generated from a light, like a halogen spotlight outside the house... if there's a heat signature coming from a spot that does not have a light, or anything else, that looks suspicious.... you can vent it through the dryer vent to make it look like you are doing laundry... or spread it under the house if it's up on a wood foundation...
ALSO, owning a grow light is not illegal. turning it on is not illegal. what is illegal is the plant. remember this, this is very VERY important.
unless you sell directly out of your house, or they see your plants, or someone signs and affidavit saying you have plants in your house, it is kind of hard for the "man" to catch you. a FLIR camera is not enough evidence. unless you are planning to run a 300 plant grow, carrying ridiculous amounts of soil and fertilizer, and living by yourself wasting like 600 dollars of electricity a month, and having a lavish, extravagant lifestyle without any sort of legitimate job.... you'll be fine.....