tray icon does not appear when activating the printer

[dino99]

Wily gnome-shell 3.16.3 session
When a session is opened, the HP icon appear in the systray for a few seconds, but then is hidden as per the setting; but does not reappear when it is expected
 (hplip 3.15.7)
This has been reported first on lp:1468624

About HP-Toolbox settings:

- Auto Refresh tab: 'Enable device auto refresh' active, 'Device to refresh' only the currently selected device
- System Tray Icon tab: Hide when inactive, Messages to show: errors & warnings

   There is a problem here:
        when the printer is powered on, the icon does not appear in the systray (like all other apps do) when the printer become active.

        I have not found warning or error logged about that issue: so either the upstart or systemd event is ignored/lost/broken or not defined.
- Commands(Advanced) tab: empty, except '/usr/bin/simple-scan %SANE_URI%' about the scan line