Good question. The exit sign is a plastic cover on top of a red light fixture. I took a look at a couple of tourist smartphone videos walking through Mandalay Bay (by the way, the corridor from the elevator bank to the end hotel room is a loooong way) to see where the exit signs are placed.
That sign would have been on the other wall on the elevator bank side of the fire door stairwell probably along the baseboard. How did it get removed and end up at the bottom of the opposite wall? Maybe the police pried it open to check for a camera or explosives behind it and tossed it aside. Or it came off somehow having to do with how the shooter supposedly 'blocked' the stairwell entrance.
The stairwell blockage is a mystery. That would have logically been the escape route for the shooter, perhaps up a few flights to an accomplice letting him into the Four Seasons through opening a fire door.
4:42 mark a similar exit sign along the baseboard: https://www.youtube.com/watch?v=ri9D7i-TZKA
>> That sign would have been on the other wall on the elevator bank side of the fire door stairwell probably along the baseboard. <<
Or the rear part of the two-faced exit sign attached to the ceiling near the end of the corridor
Thanks for the video.