From WFTV-9 (ABC):

“These searches are problematic because there is a rea propensity to use them as a pretext,” says Lars Trautman, a senior fellow of criminal justice and civil liberties policy with the R-Street Institute in Washington DC. “If you saw something illegal, or touched something illegal there is some sort of object at the end of that, that can be brought in front of a judge, a defendant can challenge it and say there is no way that is what you say it was, but you can’t contain a smell, so it’s just the officer’s word that he smelled what he said he smelled.”

Trautman is a former prosecutor and says these searches, while legal, should be documented as a way of preventing and rooting out misuse.  He also points out that the sense of smell can vary from person to person and is subjective.

“Because smells can linger, they can cling to different things and just because you smell something like marijuana in the car, doesn’t mean there’s anything there,” says Trautman.