is there any way to detect when a cocoa window has been closed, hidden and/or the user has changed the tab on which he was before?
you could use notifications to broadcast such information throughout your application:
NSNotificationCenter Class Reference
you can find some good information & examples in the iTunes U podcasts of brad larson.
this one here is a pretty great tutorial as well: http://cocoawithlove.com/2008/06/five-approaches-to-listening-observing.html
an absolut basic tutorial example can be found here: http://agilewarrior.wordpress.com/2012/02/09/dead-simple-nsnotification-example-iphone/