Rigoletto conundrum

Another interesting conundrum from Verdi, this time from Rigoletto. What is the significance of Sparafucile appearing just at that moment? Was he just touting for trade and generally in the habit of appearing on street corners asking people if they wanted the services of a friendly neighbourhood assassin. Or had he done some research and knew that Rigoletto might be needing his services. Are we to assume that the Duke, who seems to have been in the habit of frequenting Sparafucile's establishment, had dropped some sort of indication that the Duke was having an interesting liaison with Rigoletto's daughter.

Sparafucile's establishment is some sort of inn, so are we to assume that he is the sort of 'hail fellow well met' publican in whom people confide things? Is that why he is also a successful asssasin?