To some extent, I forgot about Unsolved Mysteries, which ran from 1987 to 2002, hosted by Robert Stack. It reminded me a lot of In Search Of with its documentary style format and emphasis on unsolved crimes, unexplained phenomena and missing persons. Also highlighted were mysterious legends. I recently ran across the case of Agatha Christie’s disappearance in 1926:
Here's the link for part two of Unsolved Myseries: http://www.youtube.com/watch?v=UuYlX2-Taw4&feature=related
Also, If you get a chance check out the 1979 film Agatha starring Vanessa Redgrave as Christie and Timothy Dalton as her husband. The film is a fictionalized account of her disappearance and though uneven is entertaining.