You could use the Window Closed event of the RadWindow control for this. Unfortunately there is no straight-forward way to bind your command this event.
You could also "listen" for the commands that the RadWindow control throws - the WindowCommands.Close command is thrown when the close button is hit.
Hope this helps.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.