A viridium patch
A viridium patch was a small device which could be attached to clothing and used for tracking purposes. The patch could be detected up to two sectors away.
In 2293, Captain Spock placed a viridium patch on the uniform of Captain James T. Kirk as the latter prepared to beam over to Kronos One from the USS Enterprise-A, following the assassination attempt on Klingon chancellor Gorkon.
After Kirk's arrest and trial, Spock was able to use the patch to rescue Kirk and Leonard McCoy from the gulag at Rura Penthe. Upon learning from Kirk that Spock had applied the patch, McCoy remarked, "Why, that cunning little Vulcan." (Star Trek VI: The Undiscovered Country)
In Foul Deeds Will Rise, a viridium patch is attached to the clothes of members of the Enterprise rescue party attempting to free civilian hostages on the planet Oyolo (β). This technology was developed by Starfleet and was classified at the time. A subsequently published novel, The High Country, also features a viridium patch used by the Enterprise crew, though it is set decades earlier.