When a person has been traumatized and develops a condition such as Post-Traumatic Stress Disorder they may dissociate in different ways.  One form of this is dissociating emotions, feelings from thoughts.  A person will appear robotic or with no expression (a flat affect is used by mental health professionals to describe this).  One can find this in meditative states as well as hypnotic trance states.  In this state, a person doesn't make a good liar.  The internal censor does not operate.   If you ask someone in this state a question they will answer in a matter of fact way.  In the movie "The Invention of Lying," this happens.  In that movie, they take honesty to a different level or express it differently.   In other words, you are not being dishonest if you do not share all of your feelings. 

Dishonesty, or lying, is a deliberate attempt to communicate something that is false.  That does include any communication that gives the impression that something untrue is actually true.  In any drama on TV or in the movies, when you hear the phrase, "this didn't happen" or "this meeting never happened" it is clear that you are asking someone to lie if asked about an event or meeting.  If you leave out information when discussing matters related to the meeting or event, you are following the implied request to lie.

Perhaps if you want information from a spy or someone trained to keep secrets, if you could get someone into this state of mind, where there is a disconnect between emotions and thoughts the person will forget to keep the secret, i.e. a person might even forget to forget.