I live in a suburban area where the helicopter is sometimes seen overhead at this time of year, flying slowly over the countryside just outside town. The country is quite rugged here, not very productive for agriculture and lots of dense gorse patches close to a relatively large population. I'm assuming they're looking for grows. I very much doubt they could spot the heat from a 400W light vented into the attic though. From what I understand they'd only go to the effort of pointing a thermal camera at your house if they already suspect something and are looking for grounds to conduct a search. So long as you don't live in a row of fairly new, identical houses where an unusual heat signiture really stands out from your neighbours houses I'd say you're fine, and even then 400W is very little. As long as your rooftop doesn't stand out as the only one in the area with no snow on it in winter you're probably safe enough.