A small adhesive patch impregnated with viridium, detectable by sensors across great distances, which Spock placed on Kirk's uniform before the captain surrendered to the Klingons.
The viridium patch appears in Star Trek VI: The Undiscovered Country. After Chancellor Gorkon's ship is fired upon, apparently by the Enterprise-A, Kirk decides to beam aboard the Klingon vessel with McCoy to offer help. As he leaves the bridge, Spock claps him on the shoulder and leaves a dark patch on the back of his jacket.
Kirk and McCoy are arrested, tried and sent to the frozen penal asteroid Rura Penthe, far inside Klingon space. The patch allows the Enterprise to follow them.
Once the two escape beyond the shield that blocks transporters, with the unwelcome help of the shapeshifter Martia, Spock locates the signal and beams them up seconds before they can be shot by the camp commandant.