While Laure and Richis are leaving town, Grenouille is in the Arnulfi workshop processing jonquils, in good spirits because he has 24 flacons of the perfumes of his earlier victims, and plans to possess the final scent today. He plans to sneak into the Richis house by wearing no perfume, and then sneak into Laure's bedroom that night to murder her.
The narrator is setting Grenouille up for disappointment and fear here, as Laure is slipping from his grasp unbeknownst to him. Notice that by going without perfume, Grenouille is now using his lack of scent to his advantage.
Grenouille finishes his work around noon and steps outside. Smelling the air, he knows Laure is gone and panics momentarily. Druot returns from lunch and mentions in passing that Richis and his daughter left for Grenoble that morning. Grenouille runs out of one of the town gates and smells Laure's scent coming from the opposite direction from Grenoble.
Grenouille is too powerful to be tricked by Richis's story that he and Laure are headed for Grenoble. This begins to assert Grenouille's dominance over even the very intelligent and powerful Richis.
Grenouille runs to his cabin, gathers his supplies, and follows Laure's scent south. He catches the runaways quickly. As evening comes, Grenouille goes to an inn and says he's a journeyman tanner, and they offer him a stall in the barn. Two hours later, Richis and Laure arrive at the inn. Upon hearing there is a tanner sleeping in the barn, Richis goes out to inspect the man and finds him sleeping. Later that night, he locks Laure in her room from the outside and goes to bed, satisfied with how events are playing out.
By any other metric, Richis is doing everything right to protect Laure, but it seems that Grenouille’s mastery of scent makes him practically all-powerful.