Shirley Wilkes was the secretary to Marine Captain Mike Watson and even helped bring his morning coffee.
Upon realizing that he was being held hostage, Watson slipped a note to Shirley, advising her to take the day off.
Presumably, after reading it, Wilkes headed straight to NCIS Headquarters, requesting the help of Special Agent Leroy Jethro Gibbs and his team to try and save her boss and his family before it was too late. There, she gave them the note and advised them to save the Watsons, especially the young girl, Sandy.
Wilkes's fate after that is unknown but it is assumed that she's probably still stationed at the Pentagon working under a new Captain after Mike Watson was exposed as the mastermind behind his family's kidnapping or she has moved elsewhere.