Is it possible the logograms could represent a sector of the galaxy? Not in terms of a specific planet or star, but a region of systems classifiable by stellar type?
The rings aren't all that different from the clouds that appear in Witchspace, that do vary in pattern depending on the mass and age of the star, ie. Herbig A/B and O systems.
Could someone correlate the reported sites of observed logograms to nearby stars and the cloudforms while tunnelling from nearby systems? That might tell us the Thatgoids have had something similar to the DSSA.
As Thargoid surface sites don't feature the logograms, while the (eight) Titan versions do, could this be an inferance to the 8 observable "windows" viewable from Sol? Baade's Window and the area near Betelgeuse are regions offering a clear vantage of observing the Galactic Center. The Titans could use this area to effectively track any ships in human space, to determine how high our species' available resources and ships are in number.
As to why they wouldn't consider us noticing their markings, maybe they don't perceive as we do? Ants, for example, use chemical indicators to ID each other, but they usually don't consider humans tracking them that same way...which leads again to the logograms' mystery - are they visual cues, biomarkers of a sort (since the Codex inferred electromagnetic communication)? I'm reminded of the game Oaklahoma Poker, or Blind Man's Bluff, where players keep a card on their foreheads - maybe the Thargoids are "photographing" their regions on themselves with the markings, as the "clouds" of witchspace could possibly be ejecta from stars, like extra hydrogen (tritium?) and vaporized plasma/metals...
The Thargoids, as a supposedly old species, could use the age of stars as a metric for tracking activity over large areas for a period of time; perhaps the logograms could be an extra means of reporting this?