I was able to duplicate this in my game. It happens when a resident sim heads off to work. I also noticed that there is a black shadow that is briefly placed on the floor as well when a sim uses the elevator to go downstairs. It appears to be a graphical glitch during the transition that gets cleaned up by the end of the transition. I think what happens when the sim goes to work is a failure to clean up the glitch. I think it is supposed to be the shadow cast by the sim's feet. When the sim returns home from work, the shadow is finally cleaned up. Will add to the list.
