How does Valentin finally arrest Flambeau in "The Blue Cross"?

The small priest met by Valentin on the train actually leaves clues for the police to follow once he realizes that the tall man he's met is pretending to be a priest so as to rob him of the silver and sapphire cross he carries. First, the little priest thinks the man he's with (who is really Flambeau) is actually a thief, so he switches the salt and sugar to see if the man says anything. He does not, indicating that he does not want to draw any attention to himself, and so the priest waits for him to leave, and then he throws his soup at the wall, knowing that people would talk about it and it would attract attention. Later, when Valentin discovers the salt/sugar switch, he begins to follow the little trail of oddities the priest leaves behind.
Next, Valentin finds a fruit vendor who talks about the two clerics who tumbled his apples out into the street, and Valentin discovers that two of the signs in the stall have been switched as well. Valentin then boards a bus, bringing two cops with him, just like the two clerics are reported to have done, and he gets off when he spots a broken window in a restaurant. A waiter there reports that two black-clothed clerics came in and ate and then one broke the window. The little priest had altered the bill, another test to see if the man pretending to be a priest would draw attention to the mistake; when he did not, the little priest broke a window to continue to trail.
After this, Valentin stops at the only open shop—a candy store—and makes a purchase; the woman who works there tells him that she's already sent off the parcel, so she cannot give it to him. When Valentin asks for details, she tells him about the little priest having left a parcel in her shop and how he asked her to send it along to Westminster if she found it. Valentin then heads up the same road the clerics took, eventually happening upon them. He listens to their conversation and feels, for a moment, that he may have made a terrible mistake—that is, until he hears the small priest say that he'd known the other tall man was a thief all along.
After the small priest describes what he'd been doing all day, he even identifies Valentin and the two police officers he has with him hiding in the bushes. Finally, the little priest says that he no longer has the cross with him—so he cannot be robbed—because it was in the little package he'd purposely returned to the candy shop. He'd switched the parcel for a decoy, which Flambeau took, believing it to be the real cross.