Since some time ago (and I can’t recall when this started, sorry) scsynth boots automatically when I start sclang, both through the ide and from the command line.
FWIW, I have no startup.scd.
Any clue about what could be causing this behavior, and where I should look to stop it?
(SC v. 3.10.4 on Arch Linux)
No idea really, but maybe you could creating a startup.scd and see if that changes something? Did you install from aur? If so, maybe there is a stray startup.scd somewhere in your system? Have you tried searching for this?
Sorry, I went stupidly crazy over this:
I tried both creating an empty startup.scd and searching my system for any startup.scd. Nothing there. I installed from the community repo, not AUR.
Anyway, prompted by your post, I tried a sudo make uninstall on a source code folder I have from the development version, just in case, but that didn’t stop scsynth from auto-booting either.
Then I reinstalled supercollider and sc3-plugins from pacman, and still the same.
So I removed supercollider from pacman, reinstalled the dev version, and the server is still booting on its own…
Back to pacman’s version… of course still auto-booting.
Finally, it was some code in an Extension I wrote: I had put a Server.default.waitForBoot inside an *initClass method, ingenuously thinking it wouldn’t prompt the server to boot, but just wait for it to boot on its own. I have now replaced it with doWhenBooted.
It’s probably a bad idea to have such behavior there, but at least for now scsynth is not booting on its own anymore 
Glad to hear you found the solution! Also nice to meet fellow arch/sc/Scandinavians in here