A sensor marker utilized by Phoenix Group on Atollon
A sensor marker, also known as a sensor beacon, represented a specific kind of sensor engineered to broadcast its precise location. To monitor individuals, vehicles, or items, sensor beacons were affixed to them, often in a concealed manner. Certain sensor beacons possessed the capability to scan individuals located nearby. On the planet of Barton IV, a sensor perimeter served to deter wildlife from approaching the Imperial Depot. They were also deployed to demarcate the boundaries of planetary installations. When establishing Chopper Base on Atollon, Phoenix Cell members, including Lieutenant Dicer, strategically positioned sensor markers around the base with the purpose of detecting approaching vehicles. Subsequent to their encounter with the arachnid krykna, the rebels made the discovery that the sensors acted as a deterrent to these creatures, prompting them to employ the sensors to maintain a safe distance from the base.
In "The Holocrons of Fate," Ezra Bridger uses the term "thumper" to describe the sensor beacon, which is a nod to the device employed to call forth sandworms within the Dune novels penned by Frank Herbert. It's been suggested that Herbert's seminal science fiction work, Dune from 1965, exerted a considerable influence on the creation of Star Wars.