Space buoy
From Memory Alpha, the free Star Trek reference.
A space buoy or marker buoy was a navigational marker placed in a particular location in space as a aide to travelers.
Upon encountering Balok's cube, Spock suggested that it might be a marker buoy or potentially a form of "flypaper." (TOS: "The Corbomite Maneuver")
The United Federation of Planets had a system of signal buoys in place. (TNG: "Force of Nature")
Buoys could also be used as sensor devices in a way similar to mobile probes. In 2370, as a security measure, the Federation established a series of checkpoints and sensor buoys along their border with Cardassian space. (TNG: "Preemptive Strike")
In 2370, Hekaran scientists rigged space buoys to produce verteron pulses, which ultimately disabled the USS Fleming, a Ferengi marauder, and the USS Enterprise-D. (TNG: "Force of Nature")
