This is a normal behavior for a Windows based software with docked child windows.  When you undock a child window and move it to the second monitor, Unity will still see the window as undocked and located on the second monitor.  This is controlled by the display driver and the windows operating system.  Unity has no control over it.  Even though the monitor is off, the windows display driver is still showing that the second monitor is configured and active.   The same behavior happens if you open notepad and move it to the second monitor and close.  When you turn off the second monitor and launch notepad, it will still be on the second monitor.

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