The source code button only displays if TinyMCE editor menu is enabled.
If I drag the source code button to the editor then enable the menu and save. I can then see the button in both the dropdown menu and on the editor where I dragged it to.
If I then disable the menu, because I do not need it, the souce code button is also removed.
In both cases I had to log out of word press to clear session and browser cache files to see the changes.