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.
There are cabanas on the property that could have vent windows but typically in the newer constructions they don’t want windows opening in the towers for reasons learned from the old MGM fire. Opening the windows allowed the fire to spread faster and further than it would have if the guests were not opening the windows. The newer buildings also have firebreak/containment doors that are normally held open but in the event of a fire alarm, are released allowing the closures to close of certain sections.
5/11/2010 - The vent on our window was broken so majority of our stay our room was noisy from the wind and outside noise.
https://www.yelp.com/biz/mandalay-bay-resort-and-casino-las-vegas?hrid=XMTQEHB5rr5SsJPpOxCi4A
One thing to keep in mind...A window can be "floor to ceiling" and still have a vent in it.