In Chapter 13 of Oliver Twist, Fagin is very much troubled by Oliver's separation from the thieves. This separation occurrs in Chapter 10, when Oliver picks the pocket of a "very respectable-looking personage" and steals his handkerchief. Realizing the theft, the gentlemen calls out "stop thief!" and a police officer chases and apprehends Oliver. He is arrested and taken to the magistrate (in Chapter 11), leaving Dodger and Charley to return to Fagin. 

Fagin's concern about Oliver's disappearance is about self-preservation. He is worried that Oliver may tell the police all about the criminal activities of the thieves, and for good reason. As a fence, Fagin is a person who sells stolen goods and could therefore be arrested and incarcerated for committing such a crime. 

But this concern poses another problem, as Sikes realises: "Somebody must find out wot's been done at the office." In other words, who is to enquire about Oliver at the police station? Neither Fagin nor the thieves want to go to the station, for fear of incriminating themselves. This burden falls on Nancy, whom Sikes intimidates into going to the police on their behalf.

Fagin is safe for now but it is only a matter of time before his crimes are revealed.