When there is no panel running, executing: xfce4-panel -p shows a popup window "No running instance of xfce4-panel was found", which offers a choice of starting the panel. This indeed starts the panel, but then another window is displayed that says "Modifying the panel is not allowed" because the panel is running in kiosk mode. Executing "xfce4-panel -p" when the panel is already running works just fine. OTOH, "xfce4-panel -a" does not allow starting the panel on-demand at all.
Closing old bugs