Even though Dropbox's app indicator dissapearing from the tray area seems like a recurrent enough problem, my issue is a litte different.
When I login, I can see the app panel populating, and the dropbox icon does indeed appear (config'd as startup application), but after some other icons show up (bluetooth, battery, etc.) it's gone. It's still running though.
I'm guessing it's having issues with staying pinned, and I don't know how to start addressing this problem.
I have tried many if not all solutions provided here in the forums for the "icon missing" questions.
I've whitelisted everything regarding panel
Uninstalled-reinstalled (with and without rebooting in between)
Overwritten current installation
Purged installation from terminal
Installed from Software Center, from .deb file (from dropbox website) and from terminal (commands listed on dropbox website)
batch deleted every "dropbox" ocurrences from terminal (files and folders) and reinstalled
sudo apt-get install libappindicator1, it installed, but didn't solve anything
A very important fact is that it used to work, then out of nothing it stopped syncing (a corrupted file I think). I reinstalled dropbox and the applet started behaving as I described above.
I'm on Ubuntu 12.04 LTS - 64 bits.
Any insight would me much appreciated!