Purely speculating here, but I think Parcells could be the "mystery candidate." Payton knows announcing that Parcells is interviewing would bring an unwanted media circus, so he just leaves the candidate unknown. Parcells has 3-4 credentials, and we all know his relationship with Payton.
Personally, I want Ryan, because I think he brings back a swagger to the defense that we need. What are y'all's ideas on the "mystery candidate?"