During the closing scene, several ghosts previously defeated by the Winchesters appear including Constance Welch, Mary Worthington, and John Wayne Gacy. Constance Welch was dispatched when Sam forces her to confront her kids in 1.01 Pilot, but her husband says that her remains are buried in a plot behind their old house. Mary Worthington was dispatched when Dean uses her own powers against her by showing her spirit her reflection in 1.05 Bloody Mary, and although she was cremated, she was tied to her mirror by her unsolved murder at the hands of Trevor Sampson. John Wayne Gacy was cremated and his spirit was dispatched when Sam burns his cigar box in 14.13 Lebanon.
A ghost this the spirit of a person who remains connected to the mortal realm because they are attached to it through unfinished business or an unwillingness to move on.