Dropbox under Linux supports command-line mode. After you enter Dropbox in Terminal, you'll be prompted as follows:
Dropbox command-Line interfacecommands:Note:use Dropbox Help<command> to view usage fora specific command. Status get current status of the Dropboxd help provide help Puburl get public URL of afile inchyour dropbox stop stop Dropboxd running return whether Dropbox is running start start Dropboxd fi Lestatus Get CurrentSyncStatus of one or MoreFileslsList Directory contents with currentSyncstatus Autostart automatically start Dropbox atLoginExclude Ignores/excludes a directory from syncing Lansync enables or disables LANSync
The icon for my Dropbox client is missing. So enter the command, launch & restart. Then the problem is solved.
PS: Although the icon disappears, from the command point of view, the file is still in sync state.