The pheromonic sensor was a type of sensor commonly used on explosive devices. They were triggered when a particular individual or species came within range.
Ferengi locator bombs use pheromonic sensors to lock onto their target, and Flaxian assassins were known to favor pheromonic sensors. (DS9: "The Nagus", "Improbable Cause")
In 2371, Elim Garak used a pheromonic sensor to trigger a bomb in his shop, thus implicating the Flaxian assassin Retaya on the station and instigating an investigation. (DS9: "Improbable Cause")