People always say network manager icon,sound icon,or other applets is losing from the gnome-panel and want how to get the icon back.Despite these icons’ missing may be caused by series reasons,the first solution come to my mind is add them back by “Add to Panel” in right-click on panel menu.

All these icons can be re-added onto the gnome-panel,so we just need to know what’s the applet name.See following picture:

The picture shows all applet names,you can check them by right-click on an icon and select “About”.
So if sound icon,mail notify icon,or graphics driver icon is missing, add indicator applet to panel.If switch user,shutdown,etc.., add indicator applet session.If network manager,add notification area.If date and time,add clock.