I would not be too concerned if there is an Aikido Teacher whom you "know" to be a "Shihan" that is not on the list that was posted previously. First of all, that list is from a bulletin board posting that was made on Nov 13, 2002, nearly seven years ago! Clearly, a lot of teachers have been appointed as Shihan of their own Aikikai affiliated organization since then.
Also please read again the portion marked by a * which I quote below:
* In Japan, there are many Shihan not mentioned here, who are professional Aikido instructors holding 6th Dan up allowed to use the title of Shihan according to the traditional rules existed even before the promulgation of Aikido World Headquarters' International Regulations.
This would explain the absence of people like Saotome Sensei, Yasuo Kobayashi Sensei, Hiroshi Kato Sensei, Shoji Nishio Sensei, et al from the list.
For reference, one can read the original post made in 2002 and the ensuing discussion here: