Secret

The Eleventh Doctor noted that he had kept the War Doctor a secret, even from himself. (TV: The Day of the Doctor)

Lady Peinforte described the TARDIS Wiki as a "place of secrets", in which she cribbed from to learn of the Doctor's secrets. (PROSE: Lady Peinforte)

George Thompson kept many of his fears a secret from his parents, instead writing about them in a special book. (PROSE: My Special Book)

Information from invalid sources
In one million BC, the First Doctor told the Tribe of Gum that he was unable to grant them the secret of fire but he was able to present them with something else that would warm them up a bit. This turned out to be Raquel Welch. (NOTVALID: An Unearthly Child: The Unscreened Edition)

The Evening Standard once reported on a "Cyberleader's sordid love secret" in a front-page newspaper article by Sarah Jane Smith. (NOTVALID: Doctor Who? 163)