Tanker boarded south of Trelleborg – suspected violation of maritime law

How often does this sort of thing happen? I don't mean "Swedish maritime officials impound a dodgy ship", but rather how often do vessels suspected of violating maritime law get impounded? Every once in a while, there's an article about how Russia operates a "ghost fleet" of oil tankers, or how North Korea trades meth for oil via apparently sketchy sailing vessels, but I get no sense that the more above board nations do much about those situations.