A sensor drone was a type of automated probe.

The Xindi utilized this form of technology during the 22nd century. Degra prepared to launch one to analyze the proving ground site after the failure of their Xindi superweapon prototype in 2153, but was attacked by Enterprise NX-01 before he could begin the launch. (ENT: "Stratagem")

During the early 2270s, the Federation maintained sensor drones along the border with Klingon territory. The Epsilon IX station utilized these drones, which they were able use to tap into Klingon communication channels, as was done when one of their sensor drones intercepted the Klingon's attacking V'Ger in Quad L-14. (Star Trek: The Motion Picture)

In 2373, two class-5 intelligence drones were stationed in the vicinity of Portas V. Captain Benjamin Sisko ordered that their sensor logs be retrieved in order to find out who had recently visited the planet. (DS9: "For the Uniform")