The hidden area unlocks the "Creative Space" achievement and is located in the same area as the airplane you come across and jump on to get to the lower level. After dropping from the plane, head to the left of the furthest wing and along the rock wall. There will be a slightly hidden hole. Go through and follow the path along the cliff to come to a cave with a giant typewriter. Alan will say a bit of dialog, and if you go atop the typewriter, you can read the paper which gives more insight into the plot.