A woman with dementia who went missing in Florida was found by a police dog in a matter of minutes, having bottled her scent in advance.
Citrus County Sheriff's Office said the anonymous woman had used a specialist scent preservation kit.
It can hold a person's scent for up to seven years.
In a Facebook post police said she stored the scent two-and-a-half years ago, and a picture of the jar showed it was dated January 2015.
Scent preservation kits involve rubbing a pad on a person's underarm, then sealing it in a sterile jar so police dogs have a reliable scent to smell before looking for a missing person.
Manufacturers say they work better and more quickly than articles of clothing, because they are not contaminated by other people's smells or smells from the environment.
Dogs have a stronger sense of smell than humans and working police dogs are trained to sniff out drugs, people and in some cases corpses.
Some police forces around the world, including in China and Germany, have held scent samples from criminal suspects and crime scenes to help in their investigations.
But there are concerns over a high failure rate; in 2006 it was found that only a quarter of people indicated by dogs in New South Wales, Australia, turned out to be carrying drugs when they were searched.
In this case, though, the missing person was found and the dog earned a celebratory ice cream.