My first ever first-to-unlock achievement!
There's a listening post in Col 285 Sector FG-X D1-49. It's only couple of hundred LYs from the Bubble. Drop out of SC and scan it in the usual way. Et voila.....
It gives you a message with a fragment of a communication containing two more system names and a partial planetary coordinate. I assume it's the start of a treasure hunt of some sort. I haven't followed it up yet.
One down, 49 more to go for the other achievement!
EDIT: I've found the YouTube video below which explains the "treasure hunt" and where it ends up taking you, for anyone curious. Credit to YouTuber AnubisRa for beating me to it and saving me the hard work of finding the midpoint of the three signals!