Across human space there are a handful of permanent Distress Calls and Megaships that can spawn Interceptors. These will typically have 1-2 Interceptors spawn upon arrival. Megaships also will sometimes have scouts instead.
Relogging (exit to menu and log in) will cause these locations to refresh with potentially different Thargoids.
A few notable locations:
For an extended list, check out the Thargoid Attacks Spreadsheet
Distress Call Points Of Interest that contain megaships or capital ships attacked by thargoids are leftovers of the initial bubble invasion. They have been placed there manually by FDev, who forgot to remove them once the invasion was over. Deciat specifically was attacked on July 5th 2018, and there are about 40+ more systems with the same type of POI all over the Bubble and Pleiades region.
These POIs are not an indication of any actual thargoid activity by themselves, only Non-Human Signal Sources or AX Combat Zones are. Keep in mind though, that NHSS are naturally present in Pleiades region (roughly 150Ly around Merope) and Witch Head Nebula, and do not necessarily mean those systems are in danger.
List and Project Lead: CMDR Melting Cube.
Contributors: CMDR Aranionros Stormrage and CMDR Darth Vader