As I understand it, there are an old and a new systray system in linux. If I use cairo-dock to replace the mint-panel (xfce or cinnamon for example), I will have almost all tray icons because they use the "new" systray (network, sound, dropbox, ...). However mint-update seems to use the old systray. Cairo-dock has a "old notitication" applet and mint-update shows in it. However it is not as stable as it should. To improve compatibility, every mint-specific tray icon should use the new systray.