In To Kill a Mockingbird, what does Dill see on the Radley porch?

Dill, Scout, and Jem decide to spy on Boo Radley. They wait until it is dark outside and they quietly sneak onto the Radley property. They open the creaking gate and step inside the fenced area, which every other kid in Maycomb is terrified to do. Dill, Scout, and Jem walk around to the rear of the house and see the Radleys' back porch. The porch makes the house look abandoned:

The back of the Radley house was less inviting than the front: a ramshackle porch ran the width of the house; there were two doors and two dark windows between the doors.  Instead of a column, a rough two-by-four supported one end of the roof.  An old Franklin stove sat in a corner of the porch; above it a hat-rack mirror caught the moon and shone eerily (To Kill a Mockingbird, Chapter 6).

The children run into some chickens as they prowl around the back yard. Dill peers into a window on the side of the house, but he cannot see anything except for curtains. Dill suggests looking through the back window, but Scout protests. He goes around to the back anyway with Scout and Jem following. Jem quietly climbs onto the run-down porch and looks through the window. Then Scout notices something:

Then I saw the shadow. It was the shadow of a man with a hat on. At first I thought it was a tree, but there was no wind blowing, and tree-trunks never walked. The back porch was bathed in moonlight, and the shadow, crisp as toast, moved across the porch toward Jem.

After this, Dill sees the shadow,too. Afraid, he covers his face with his hands. Jem is the last one to see the shadow. Terrified, the children run as fast as they can to escape.