If you are using more than one monitor to improve your visual 3D real estate in Rhino 5, then you need to get used to the loss of Panels location, size and existence.
I am forever replacing, re-positioning and organizing the Panels that I have placed in my second and third monitors, because Rhino 5 forgets them, or sometimes just some of them.
I have not seen any full solution to this issue, and for that matter no intent to implement a solution.
Will this issue be solved? With the design of something like “work-spaces” that a user is able to save and recall?