Perhaps there's a vortex of some kind on the 40th floor, where you change elevators to continue up. Perhaps they've discovered a method of invisibility and are waiting to see how long we take to notice. Maybe someone started a game of hide-and-seek and we just didn't find out. Maybe they went home with the alien death flu.
I'm sure there's some deep philosophical answers to be found in all this, like 'the meaning of life', 'the nature of man' or perhaps 'why you shouldn't go to the sandwich shop at Fourth and Cherry'.