If we are really taking about a system tray icon, the one possible answer could be the fact that unity (under 14.04) is not showing the system tray icons at all.
The one thing that works is to add a special PPA and fetch the "unity" from there.
This version of unity is patched (basically it is the latest unity + the code for showing the system tray ... the part that was removed by Canonical).
The one problem: it could happen that the official PPA has newer versions of unity; and if you dont pay attention, the newer version is installed at some point; and whoops, next login ... system tray is gone again.
If we are really taking about a system tray icon, the one possible answer could be the fact that unity (under 14.04) is not showing the system tray icons at all.
The one thing that works is to add a special PPA and fetch the "unity" from there. This version of unity is patched (basically it is the latest unity + the code for showing the system tray ... the part that was removed by Canonical).
So, you can try:
Reboot (or logout and login again).
The one problem: it could happen that the official PPA has newer versions of unity; and if you dont pay attention, the newer version is installed at some point; and whoops, next login ... system tray is gone again.