Have never seen such a setup in a map before, although it is surprisingly easy to do (credit goes to UniqPhoenix for the simplification of my own method).
Create a Door with wait = 1.1 second (the speed at which door moves must allow the door to fully open before this time)
Create a trigger_buildable with wait = 1 second (should be smaller than wait of the door, then specify which buildables you want it to trigger from)
Target the door with the trigger.
Thats all.
When the buildable is touching/inside of the trigger brush, it will cause the door to be constantly triggered. When the door is fully open and has been triggered, the wait timer resets on it and causes it to stay for another 1.1 seconds. If the building is destroyed, the door is not triggered anymore, and it returns to initial position after short period of time.
Quite simple and can add a nice touch to a map.