I'm asking not to be difficult, or to say there was another shooter on a lower level...I'm asking because I haven't stayed in a lower level Vegas hotel room, nor at Mandalay Bay before so I don't know.
However, these two different room reviewers (from years prior to this shooting), seem to indicate in their reviews that there is at least some kind of vent opening in the window(s) of the guest rooms they stayed in at Mandalay Bay.
If that's in fact true, there is the possibility then for someone to shoot through the window vent and not break up the window.