I think it depends on how they are docked. If they are docked as a tab group, only one control pane would have been visible when the group pane was closed, so only one control pane will have the PaneHidden event fired for it. You might want to try handling the BeforePaneButtonClick or AfterPaneButtonClick. If the PaneButton value of the event args is Close and the Pane is a DockableGroupPane, you can iterate the group's child panes to know which panes are closing. If the Pane is a DockableControlPane, only that pane will be closing.