They only need a warrant to use optical systems that can actually see through walls. FLIR cannot. FLIR merely picks up heat signatures on the surface of the outside of your building.
http://www.rollitup.org/legal-edge/1...st1674031.html
Already quite a bit on FLIR in that thread.
The other technology out there that it is
assumed they are using, because the law in the US does state they need a warrant to use imaging equipment that sees through walls, and FLIR cannot see through walls therefore they dont need a warrant to use it, must be a microwave imaging system currently used by the military - as mentioned toward the end of that thread I posted above.
Microwave is not looking for heat signatures, it can penetrate through building material that is not metalic and feedback an image of the layout inside the room it is viewing. Building fire proofing paper usually takes care of that since it is lined with foil.
The supposed IR blocking variant on mylar being sold out there actually blocks microwaves nicely, but, does not block FLIR. Why, well because FLIR can't see through anything anyways, and heat still travels through the mylar, since that is the function of mylar, to reflect more light than heat to your plants. Where does the heat go? thats easy, through the mylar and to whatever is behind the mylar.
So assuming your local police do not have one of those microwave units, you really only need to focuss on not creating heat spots on the walls and roof of your shed.
If you have no other place to vent your heat, as in underneath or no roof cavity, then I assume there is no insulation on your roof, if so then I would suggest you not grow in that shed if you are concerned about FLIR. FLIR mounted under helicopters will pick up the heat signature on your roof if the only thing between the outside air and your lights is a tin roof.
Failing that if you still intend to grow in there, put an outside light on your shed and vent right beside it. FLIR would be expected to show a heat signature around the light. You could also run your lights on cycle during the day, therefore lights off at night would mean a lot less heat to vent.