Sure, I'm a fan of Found Art and the readymade, but I'm becoming more and more interested in found performance art (see William MacGonagall post below). According to Jacksonville News, 18 year old James L. Harris has been arrested and charged with stealing three public transit buses, dressing as a driver and driving the buses along their routes. He picked up and dropped off passengers along the way and returned the buses at the end of the day. He did not attempt to steal the money from the fare boxes.
Were it an intentional artwork, it would also fit in my my favorite types of interventions - invisible ones. When I was on the board of the 7a*11d International Performance Art Festival the best proposal I ever saw was by an American artist named Chris Wildrick. His piece was that he would come to Toronto and spend the day dressed as a superhero that he created, but dressed as that superhero's secret-identity alter-ego.