That Moment

Looked out my apartment window, and at that moment, a shiny, black bmw mini suv was driving north, somewhat slowly as the driver was trying to navigate a biker and make a left turn.  Something on the car’s roof caught my eye as the car was turning. I moved to the other window to follow the turn and identify what I had seen.  On the roof, a notebook/soft briefcase, waiting to slide just a little closer to edge and off into the street.  My window was closed and too far away to yell, and apparently no one else was seeing it.  I have no idea what happened as the car moved out of sight.  Did the passenger place it there while [s]he removed a coat before entering the car?