The web UI has an ‘automatic upgrades’ option (filed under Actions → Settings → General). The option is greyed-out. I do not know why. For (cf. this issue of mine on the bugtracker):
I did not compile Syncthing with updates disabled (indeed I did not compile it at all);
the computer in question runs Windows, which does allow automatic updates;
I have not set the update-disabling environmental variable;
I did not use the ‘advanced’ settings (Actions → Advanced) to set the update check interval to zero;
I did not start Syncthing with a flag that disables automatic updates.
I did, a while ago, disable automatic updates on the machine in question. But I cannot remember how I did it and thus I cannot manage to undo it.
In Windows, can you go to “Settings → System → About → Advanced system settings → Environment Variables” and check if you haven’t got the STNOUPGRADE variable set? Please check both in the user and system variables.
Can you also describe in details how exactly have you set up Syncthing to start automatically? I’m assuming here that you’re not launching it manually every time.