Our Ubuntu workstation is required some applications installed for complying security policy. Those applications will show some icons on Systray. After upgrade from 12.04 to 14.04, those icons are missing.
The reason is “Unity Notification Area (Systray) Whitelist Is Obsolete [Ubuntu 13.04 Raring Ringtail Changes]“. After a little bit searching, here is the solution:
sudo apt-get install indicator-systemtray-unity
Then log out and log in again.