The offer and the reluctance that is not really hidden draws a wry smile. "Not necessary," he answered. "You have more important things, no doubt, to be giving your time to and I don't need a nursemaid."

Flippant as the remark is however, it's delivered with a nod; he understands why Costigan is asking. But the thief won't require that offer to be delivered upon; the strong refusal absolves them both of the responsibility.