Some cities/counties seem to put more time/money/effort into it than others and do a lot of searching. Others seem to do none.
Some will only target big grows and grows on federal/state property.
I've never seen them do it personally, but I've seen it on tv and heard plenty of stories from other growers. Even here in California where I'm perfectly legal I get a little jumpy when a helicopter flies over.
Sounds like you've been safe so far, but if you're in a non-medical state maybe a greenhouse or carport would be a good idea. You'll be harder to spot from the air, and you'll have the added benefit of protecting your plants from rain, wind, frost, etc.