A sensor drone was a type of long-range sensor probe deployed by the Federation in the 23rd and 24th centuries.
When Epsilon IX first detected signs of an engagement between three Klingon K't'inga-class battle cruisers and the V'Ger entity in 2273, they sent out sensor drones to visually monitor the remainder of the engagement and send images back to Starfleet Command. (TOS movie & novelization: Star Trek: The Motion Picture)
In 2376, Ro Laren thought about deploying automated sensor drones between B'hava'el and the Badlands. (DS9 novel: Bajor: Fragments and Omens)
Sensor drones were also used by the Ferengi Alliance. In 2355, sensor drones were sent out by the Seventy-Fifth Rule to survey the planets of the Maxia Zeta system. (TNG novel: The Buried Age)