I've been on tailgates and stairwells (C 130, DC 9, CASA 212, Short Skyvan, etc) and can confirm that they are not turbulent locations in flight. Quite calm in fact. I was surprised when I first experienced this.
Either Boeing or NWO states in the 302 believing the pressure change caused the placard to escape. I don't find this to be plausible?
Even if the placard was placed on the exterior of the fuselage, it would probably be well within the boundary layer at the rear of the fuselage and not subjected to the full free stream air velocity. The boundary layer at the rear bottom of a 727 fuselage, and near the stair cutout, would probably be something on the order of two inches thick. In theory, the air velocity at the metal surface would be zero. But theories and facts sometimes differ.