This happens to me as well recording WPF applications under Windows 8.1 when I un-docked the recorder using 3 monitors. I submitted a support ticket about it but they were unable to reproduce the steps I provided to make the recorder disappear.
I have been able to get the window back by doing the following...
1. Unplug 2 of the 3 monitors so that I am only using 1.
2. Start a new recording session.
3. Selecting the recording window using Alt + Tab shortcut (it cycles through your open windows).
4. Then using the StartKey + LeftOrRightArrowKeys shortcut to snap the recording window back to my screen.
5. Then re-docking the recorder.
The recorder disappearing is not expected behavior from a user standoint and it would be great if Telerik could reproduce this issue.