The End in the Beginning - S4-E26
Corrected entry: We learn that Agent Booth has a brain tumor that has been causing him to hallucinate. One of the hallucinations mentioned was Parker in "The Hero in the Hold." At the end of the episode, Dr. Brennan both sees and speaks to Parker in the cemetery. Agent Booth believed Parker to be a ghost, and we were to believe that Dr. Brennan had unknowingly had a paranormal experience.
Correction: The implication of that episode was that Parker's ghost was real, since Booth points out numerous things he does during his escape that he couldn't have done on his own. Dr. Brennan seeing Parker at the end of the episode confirms this. Dr. Brennan is the only one to posit that Booth's belief that he saw Parker was a hallucination brought on by his brain tumor, because she did not learn who Parker was when talking to him in the cemetery; Booth was looking the other way, all Brennan knew was that a young man walked up to her and said something about enjoying life. Even if she saw a picture of Parker at some later date, she probably wouldn't recognize him.