A sensor drone was a type of automated surveillance probe. Sensor drones were utilized by the Xindi and the United Federation of Planets.
History
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 2364, during the neural parasite invasion, Starbase 045 was ordered by Commodore Anna Dixon to cease its monitoring of a sector. Remote sensor drones would be substituted for this outpost. Data acquired by these drones would be transmitted directly to Tactical Analysis Division. (TNG: "Conspiracy")
In 2364, during the neural parasite invasion, the starships USS Non Sequitur, USS Tranquility Base and USS Max Plank all received orders from Commodore Joshua Jae of Starfleet Mission Operations Division to modify their defensive coverage of the neutral zone by pulling sensor drones from active duty in some sectors and to increase drone surveillance in others. The USS Minnow also received orders concerning drone deployment during that year. (TNG: "Conspiracy")
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")