(covers information from several alternate timelines)
A monitor station was a Starfleet outpost which monitored communications transmitted from Federation and Klingon Empire sources. One of the tools available to these space stations was sensor drones. These stations were located in Federation space.
In the mid-2270s, the monitor station Epsilon IX intercepted a Klingon transmission from Quad L-14, which was the first indication of the threat that V'ger presented to the Federation. This station continually updated Starfleet Headquarters with information until its destruction by V'ger. (Star Trek: The Motion Picture)