I found this interesting video where they remove windows from the Mandalay Hotel. The interesting part is at 1 minute 40 seconds.....[www.youtube.com]
Obviously the people doing the windows are experienced. We also see in the video other windows removed in other sections of the Hotel. What is interesting about this point is it must be easy and a short installation time since they don't put up any type of temporary cover when removed. I'm not presenting this is what happened. But it is interesting that other rooms could have been used by shooters and then hidden by replacing the window.