From this, we can conclude: the art thief must know that Castor had an informant (at least when taking the quest, but ideally when completing it as well; otherwise, how can one be sure that the quest was accepted and not rejected?). The art thief must be in many places at once or have good surveillance capabilities to monitor... well, basically, all locations in the city. After all, they could be searching for paintings anywhere, even in the sewers. It might not be true, but it should be considered as a possibility. The thief must know the city like the back of their hand and, at the same time, never arouse suspicion from the authorities – otherwise, the police would put a suspicious person under surveillance, and that would be the end of it. It is likely that the thief is a representative of the authorities, but not of the lowest rank, and has good administrative capabilities.
Therefore, the conclusion is: the art thief is Castor L. Tselen./