Lets say you got a 20' x 96' greenhouse - the kind with the white walls. Think anyone would care / bother you? I mean, helicopters / planes would see it, but they wouldn't just assume it was full of pot would they? And they would need a warrant to search it?
Possible. . .yes. This kind of thing is done all the time.
Easy. . .no. As already mentioned, growing a greenhouse full of ANYTHING (even tomatoes) is hard work.
Yes, in theory law enforcement would need a warrant to search a greenhouse if on private property
In practice, if they're suspicious its not that difficult for law enforcement to come up with some pretext to come on your property and look around. (IE "we're looking for a fugitive"). Inspecting a free-standing structure that is outside your main dwelling would be legal if they had a reason to be present otherwise. If while conducting some such search they came across your "unlocked" greenhouse they could look inside. If they could jimmy/pick the lock, it wasn't locked. Maybe they could peer inside a crevice, or smell weed inside, etc. Basically, once police are on your property, anything they can see or find is fair game in terms of evidence.
Even if they discovered the weed farm illegally, you can bet the police won't just let you keep it. That a big bust for them, and they'll figure out a way to make it.