Hello ,

In order to be notified when the PaneButton was clicked, you should handle BEforPaneButtonClick event and to check  if the clicked button is close button. On the following link you will find simple tutorial of how to do this:

http://help.infragistics.com/Help/Doc/WinForms/2012.2/CLR4.0/html/Infragistics4.Win.UltraWinDock.v12.2~Infragistics.Win.UltraWinDock.PaneButtonEventArgs~Button.html

Please let me know if you have any further questions.