SUMMARY: 3.5.10 is not fully xdg compliant and that is why I see no related messages. Trinity is mostly xdg compliant and that is why I see the messages in the log. Yet for some reason Trinity is not honoring the Hidden=True directive in the ~/.config/autostart/hplip-systray.desktop file and I still see the HPLIP messages when I should not.
You are probably correct. I know the section of code that is causing the problem, but am hesitant to change it with only 13 days until release, as the potential for breakage in obscure ways is very large. I think I will just make this a known 3.5.12 issue and fix it for 3.5.13, as it will not affect very many users as-is (you are the first out of hundreds of known users to notice it).
Tim