the helicopters can ONLY read the surface temp of your roof... so if you arent growing in your attic your fine.. if you are growing up there you should get some IR blocker and wrap the entire attic with it...
otherwise the helicopters look for flurs, or spouts of hot air, being emitted from your house.. such as a dryer vent.. there are distinct differences in a dryer flur and a grow room flur.. size and time length are the two biggest factors.. if the see a questionable flur they will start investgating it more.. they might start flying by and keeping track of times the flur is visible to their IR readers.. to see if there are any patterns.. ie on for 12 off for 12... if they see patterns the might send a patrol officer to snoop around a little while you arent home to look for light leaks or see if they can hear fans or other running equipment.. or send them to your door to ask some stupid question about a missing dog or some shit to see if they can smell anything... then, if that all plays into their hands, they will get a warrant.. but unless your roof temp is noticeably hotter than your neighbors, or they find some sort of other evidence, they cant do much about it...