Muzzle flash from one of the towers is what was seen as a reflection from the 4th floor, Mandalay Bay, mirror-glass window theory:
https://vimeo.com/239195836

UPDATE 4 OF 10 – MFA LAS VEGAS SHOOTINGS – Towers Line Up Perfectly With Reflection Of 4th Story Muzzle Flash

MFA LAS VEGAS SHOOTINGS – Update 4 of 10 MFA CEO “Scott J. Binsack”Towers Line Up Perfectly With Reflection Of 4th Story Muzzle Flash On Mandalay Bay Glass!!