At the beginning of The Da Vinci Code it is insinuated that Captain Fache believes that Robert Langdon killed Saunière because of the message left on the floor "PS Find Robert Langdon".
The same scene also shows Fache wearing Seal of the Holy Cross and Opus Dei which is obviously meant to be some sort of foreshadowing of the real reason for his hunting Langdon.
However later in the film after Langdon & Sophie escape on the plane Fache tells Lieutenant Collet that the reason he believes that Langdon is the murderer is due to Bishop Aringarosa contacting him and supposedly breaking the Seal of the Confessional by claiming that Langdon confessed to the murder.
I could understand the message being used as an excuse by Fache, however this does not explain how the Bishop knew about the message and was able to find a high ranking member of the Sect to trick into helping him.
So the question is how does the Bishop find out about the message and Langdon's involvement seemingly before the body is even found?