Hi Daniel,
Thank you for contacting Syncfusion support
We have checked your query "how to detect close event for non-document controls". In SfDockingManager control we don't have direct event to detect non - document window which are closed. But we have prepared the sample that tries to meet your requirement. In this sample, we have triggered the close button click event to detect the non - document window which are closed. Please download the sample from the below link.
Sample:
128191_ClosingEventRegards,
Marimuthu S.