I don’t know, I have never seen that before. Our icon should always appear and we have no option to hide our notification area icon (but we do have the option to show notifications on that icon or not).
I have noticed with Windows sometimes that the notification area has some issues refreshing and updating. Then if I mouse over that area sometimes it refreshes correctly. If you mouse around that area does our icon appear?
If you reboot does it makes any difference? What Windows version is that, is it Windows 7?