Comment 8 for bug 37909

I confirm this, happens to me also in Xubuntu gutsy

the system tray icon disappears and a blank space will fill in its place. that happens mostly after i add some other items to the system tray, or when i check the options of any application thats already in the system tray.

the blank space can be removed by removing the system tray from the panel and adding it again, that way will insure keeping your system tray organized and "blank space" free without the need to restart.