In 2002 a woman tried to carry marijuana into a Brisbane prison inside a balloon smeared with coffee grounds, pepper, and petroleum jelly. The dog on duty found it anyway. This episode explains why masking a scent fails: a dog’s nose splits each breath, sending part of the air to a chamber lined with hundreds of millions of receptors, and it reads a mixture as separate ingredients where a human smells one blend. That ability now finds far more than drugs. Dogs locate quagga mussels on boat hulls, live bed bugs and viable eggs in hotel rooms, the scat of endangered animals, and even killer whale feces floating on open ocean.

Yet legal briefs filed with the U.S. Supreme Court claim that up to 80 percent of police dog drug alerts in the field turn up nothing. The nose is not the problem. Dogs read their handlers, and they also alert to traces left days earlier. The episode follows that gap between biology and law, from American traffic stops to the mass strip searches that followed dog alerts in New South Wales.

• In a 2022 Paris study, dogs sniffing underarm sweat samples identified symptomatic COVID cases with 97 percent accuracy and asymptomatic cases with 100 percent.
• The Clever Hans effect, named for a German horse that seemed to do arithmetic by reading its trainer’s tension, explains how an officer’s suspicion can cue an alert.
• In 2012 three Nevada Highway Patrol officers sued their own leadership under the RICO Act, alleging dogs were trained to alert on a handler’s signal.
• U.S. rulings in 1983 and 2005 held that a dog sniff in public is not a search. Justice David Souter dissented, calling the infallible dog a legal fiction.
• In New South Wales, drugs were found after only about a quarter of dog alerts, and wrongful strip searches at music festivals led to a class action filed in 2022.