I don't understand the question about the "Crayons of Time". Asking if they "hide a secret" looks quite generic to me. Might as well ask if any story told by characters and not shown hides a secret.
Was there, originally, a specific reason to wonder and what was the doubt about, exactly? Just that the people telling this story were lying? That there was a hidden message inside the "Crayons of Time"?

PS: What is a "posthomonous" and where can I buy it? Also, who is Girad?