RadPanelBaritem's expander arrow should not be visible when there are no subitems.

XAML Team has recently reviewed the status of this item. It is considered a design limitation of the RadPanelBar control which won't be addressed in future releases.
However, users will be able to use the ExpanderStyle property of the RadPaneBarItem (from Q1 2016 SP) in order to control the arrow's visibility.