I've found this LT 3 Mission in my episodes list. However, the only text is "Secure old Klingon Facility".
I have NO idea what system or sector this occurs in. Perhaps it was a follow up ground mission to a space mission I finished. No idea. However, not having the System and Sector makes it impossible for me to finish it. I also did a search on the forum and did not find any info.
When you come across a mission like this, it originated in a sector of exploration. When you explore a system which is uncharted, you enter into the anomalies which are the planets that you have to patrol. If you enter into one, but decide not to stay and finish the mission, then the mission is sent to your que. Notice that these missions are under the heading "EXPLORATION" in your journal. To find the planet, just go back to the sector or cluster that you were exploring at the time, and cruise around a bit. A notice will come up on your screen and ask you if you would like to continue the mission.