Wouldn’t you expect the front desk to get calls upon guests hearing massive windows being shattered?
Yes... or someone seeing or hearing it crashing outside. Regardless, it was apparently a door alarm that put a security personnel on that floor at that time.....
It was Sunday night, not too late. Maybe many rooms were vacant. Maybe people were downstairs being busy gambling. There was a concert which may have muffled the sound of breaking falling glass. It was night, so I doubt that anyone looking out their windows would have seen falling glass.