helicopters are usually just visibly searching for outdoor grows its not that difficult to spot cannabis plants from the air depending on how large the patch is, the plants do not give off any heat.
from what ive read reflectance can be used but its not in very wide use at this time, with cameras mounted on planes large areas can be covered and while im sure they are a few plants with similiar reflectance to cannabis it can pin point potential spots that they would need to search.