Listening posts are deep-space probes with signaling technology. They're primarily found in systems on the frontier of the bubble. They don't have any known defenses.
A Listening post will appear in the navigation panel when your ship is within 1,000 Ls. They can be found near primary and secondary stars, planets and moons.
While exploring a system you can perform surface scans and check the Navigation Panel for signal sources. When you found a signal source of a Listening Post you can lock on the target and drop out of supercruise.
After targeting the listening post you can download data with your ship scanner. It will show up in the Comms Panel. The data is usually packets with the same signal identifier and an A, B, C suffix. The packets shows the systems where the other listening posts are located. You must visit all related Listening Posts to receive all the location data.
CMDR Scott explains: 
- Packet A contains the longitude co-ordinate of the signal source.
- Packet B contains the latitude co-ordinate of the signal source.
- Packet C contains the body designation of the signal source, e.g. “AB 1 c”.
"To identify the source system, each packet gives a minimum and maximum range from that Listening Post. The recommended approach is to bookmark the three systems containing Listening Posts, and use the galaxy map to select potential systems. The three systems form a triangle in space, and the source of the signal is within that triangle. Rough estimation based on the ratios of the three ranges is good enough."
"Most of the time there is only one system within the correct range of all three stars, but where multiple candidate systems exist, the body designation is sufficient to eliminate incorrect candidates. Rarely do multiple systems next to each other have a landable body with the same designation."
Sirius Corporation has deployed hundreds of listening posts throughout the galaxy. A Sirius spokesperson said:
"Sending pilots into the void is fraught with danger. These probes can go further and faster than a human pilot, at much reduced cost and with, I daresay, more reliability. We've been working closely with the Mars High Astrocartography department on the probes' design."
"Human pilots are interested in sightseeing, chasing myths and 'making their mark' on the galactic stage. They get sidetracked. Our probes are immune to such distractions. At present, galactic exploration is conducted in a sporadic and improvisational way. Soon we will have far better coverage of the galaxy."