If you use Gnome 3 under Arch Linux with ownCloud client you will see no tray icon by default.
Here are the steps to get to the tray icon like you see in the screenshot
- Install gnome-Tweaks
yaourt gnome-tweaks
- Install the Appindicator extension
yaourt gnome-shell-extension-appindicator
- Open gnome tweaks and enable the extension
- Restart ownCloud-client or login and logout