I'm running Linux Mint, a dist based on Ubuntu, using the Xfce desktop environment on one of my machines. Every now and then the nm-applet, the system tray application that helps connect to and show status of the network, disappears from my panel after a system upgrade.
It seems that nm-applet must be launched in a session bus instance by
dbus-launch. This can be accomplished by changing this line in /etc/xdg/autostart/nm-applet.desktop in the [Desktop Entry] section:
Set it launch nm-applet by dbus-launch:
Log out and in again to make it take effect. The nm-applet should show up on in the panel (circled in red here):