i remember a cop coming to our elementary school and telling us this...
if a cop hears a scream, and goes into the house and a women is getting beaten, then beater goes to jail, case closed. but if he comes in the house and see's the ladies poodle took a huge liquid shit on the carpet, and next to that is a pot plant, there is nothing he can do. that was 20 years ago or more though
If the officer sees an illegal activity/substance from a position where he's legally allowed to be, he has the right and duty to investigate further. If he's legally in your house responding to a disturbance (which doesn't require your permission) and sees a pot plant, its fair game.