Killian's son said (in his first Hannity interview) that Mapes had called him aprox. 2 weeks prior to the telecast. She "had heard" those documents exsisted and wanted to know if the family had them.
That sort of fits--Burkett may be telling the truth on this one. Mapes was looking for them.
According to that Amer. Spectator article, an opposition research staffer for the DNC got these documents months ago and passed them on to the Kerry Campaign. They knew they smelled. That's probably how Mapes heard about them.