Take for example the smell of raw weed. (And let's leave aside, for the moment, the argument over whether it should be legalized or not.) I have no trouble smelling any appreciable quantity of that and if I am in a place where any Joe has a right to be, e.g. on the sidewalk, then that becomes part of PC to initiate a search if I can localize the source and articulate the facts. Maybe the search is with a warrant, maybe without if it meets the court defined guidelines and is necessary. (I'd never conduct a search without a warrant just because I could. That gives the defense one more reason to raise challenges and doubts over your justification due to 'exigent circumstances' and maybe get a righteous case tossed out.
Let's take a less benign example, like the smell of gasoline on the person of someone hightailing it away from an arson scene, or the stench of a rotting corpse in the trunk of the car. I've dealt with both. In both cases, I was where I had a right to be, where any citizen has a right to be. In both cases the perp brought the evidence of the crime out into the open, in a public place, where I could detect it. Their rights remained intact, and I got my evidence and secured convictions.
We both know that dogs have searched citizens’ vehicles w/o the presence of the odor of raw weed. Many times police have used dogs to search citizens’ cars based on “suspicion” and nothing more.