I have Ubuntu 12.04 and today when I started my laptop I saw that the Dropbox icon is missing from the top panel and I don't know how can I get it back.
This will work for versions 12.04 and above in case after installing Dropbox from their site using the DEB package, the icon does not show in Unity.
Remove Dropbox if already installed
sudo apt-get remove dropbox
Install the nautilus-dropbox package
sudo apt-get install nautilus-dropbox
In the case the steps above still do not work do the following (This package should already be installed but just in case):
sudo apt-get install libappindicator1
You will now need to restart Nautilus. You can either logout and then login or simply Reboot the PC.
The end result should be something like:
Tested on 12.04, 12.10, 13.04 and 13.10.
dropbox stop && dbus-launch dropbox start
It looks like your Dropbox preferences have not been set to: "Start Dropbox on system startup".
If you check that selection, once your system starts, Dropbox will be loaded when Ubuntu starts up, and you will be able to see the Dropbox icon in tray.
Quick check: Try starting Dropbox manually. If you manage to do, you will be able to see the icon in tray for now.
Be sure to check the above option, for if you fail to do so, the tray icon will not be persist after a reboot, unless of course you manually open Dropbox each time after the computer has started.