Any system, anywhere in the galaxy that has a Gas Giant w/Ammonia Based Life and landable planets will spawn Non Human Signal Sources on the planetary surface that you can go down to and collect sensor fragments.
Edit: Doing a simple FSS scan of the system will not reveal the Non Human Signal Sources at the landable planets. The cmdr must travel to each planet or moon to have the POI show up. The first time you visit the planet or moon you must perform a DSS, but when revisiting a system simply proximity to the planet reveals them. These POI are randomly spawned, so if you leave a system and return they will not be at the same locations or planets. If you land beside a Thargoid sensor and log off, when you log back on it might be gone. The probablity of at least one planet out of ten having a POI is very good. Some planets will have two, maybe three POI. A system with +10 landable planets will likely have several POI.
This is awesome, exactly the sort of logic that had to be there somewhere, a gold mine! I'm going to have to do some more exploring, to get just a little of this level of understanding of the system.