Third person is a temporal mirror. Using camera recognition technology, it replaces realtime video of people at the installation with clips recorded earlier. In the present, people’s movements are ‘averaged’, a little like a Muybridge photo sequence, so that they form the canvas onto which the previous footage then plays.
People are visible only as dark silhouettes against which, perhaps, you can see a previous clip of a figure in white descending the same staircase.