The ninth century monk Notker the Stammerer has an interesting take on the discovery of the plot to overthrow Charlemagne in 794:
This son of Charles had been plotting the death of the emperor with a gathering of nobles, in the church of Saint Peter; and when their debate was over, fearful of every shadow, he ordered search to be made, to see whether anyone was hidden in the corners or under the altar. And behold they found, as they feared, a clerk hidden under the altar. They seized him and made him swear that he would not reveal their conspiracy. To save his life, he dared not refuse to take the oath which they dictated: but, when they were gone, he held his wicked oath of small account and at once hurried to the palace. (From The Monk of Saint Gall: The Life of Charlemagne, Internet Medieval Sourcebook, Fordham University Center for Medieval Studies)
So, let me get this straight, a group of men bent on treason and murder would release someone who overheard their plot merely because he promised not to tell? While a medieval audience might believe the conspirators would not wish to desecrate the church (and anger God) by spilling blood within it, I think even they would have a hard time believing the would-be killers would simply walk away from the clerk under those circumstances.
Medieval people believed oaths, especially those sworn on a saint’s relic, were sacred, and breaking them could bring divine retribution. But they had seen people break those oaths and shift loyalties. Even the conspirators would know oaths made under duress were invalid.
This is another reason Notker strikes me as the kind of guy who doesn’t let the facts get in the way of his story.
In the real history, Fardulf, a Lombard deacon and scholar, revealed the plot against Charles by his eldest son. Exactly how he discovered that plot is a mystery in the reliable sources, so I borrowed a tiny bit from Notker’s account for a scene in Queen of the Darkest Hour. Just enough to be believable.