It's unreasonable search.
Say you get pulled over for a busted tail light, and the cop notices a corpse in your back seat. That's OK.
Say he says 'Ho-lee sheeeit, smells like dead body. Pop your trunk open.' And hey, there's a dead body in the trunk. That's OK.
But he can't say 'I done pulled you over for a busted tail light, but I'mma search your car for a corpse, even though I have no reason to believe there's any corpses.' Not reasonable.
Now, this guy gets pulled over for lane swerve. Fine. Cop can sniff his breath, look for signs of intoxication. Cop can eyeball the seats through the window, the ashtray, looking for booze bottles, roaches, whatever. But he can't say 'I have no real reason to, but I'm turning this traffic stop into a drug stop, *but first I need to call in extra equipment.* That's unreasonable.
If he'd happened to have had the dog with him, and decided to have the dog give the car a once-over, fine. Although I question the validity of dog searches; we know that animals can pick up on clues to what their owners want. See the Clever Hans phenomena. If the cop wants to search the car, the dog might just pick up on that and alert.