Being a lifer himself, Sal understands that some small aspects of routine are important for a child like Jennings. Throughout the complexities of consciousness for a child like Jennings, behaviors are repeated and thus routine becomes important. There are some basic routines that represent the bane of one's being because so much of life itself is unpredictable. Sal knows that Jennings has already ran away once to the zoo when it seemed like the world was closing in on him. Sal recognizes this pattern of behavior in Jennings' actions. It is for this reason that it makes sense that Sal would be there at the zoo when Jennings runs away at the end of the narrative. Jennings ran away to the zoo earlier waiting for Sal when he was transferred to the Bronx bus route. This same pattern is what enables him to understand that Jennings would be there.
Another reason why Sal knew where Jennings would be exists in how there is a connection between both characters. Sal is the one character who shares a connection with Jennings throughout the narrative. It is one in which he understands better than anyone else what Jennings endures and helps to provide for him both physically and emotionally. This sense of intense connection between both characters could be seen as a reason why Sal knows where Jennings is at the narrative's end. So few have understood Jennings. It makes sense that there would be at least one person who could understand and relate to Jennings enough to know where he is at a moment where it is sorely needed.