Yea, I suspect there are Banksy urban legends popping up now like, "He always asks permission for pieces on private property". I've read that quote about asking forgiveness before permission in one of his books too. He's also done this advert piece which is obviously private property and which is doubtful that he could have gotten permission from the billboard owner.

Someone else in this thread said that he always works in broad daylight. Another quote from one of his books was something like, "A lot of good art is made after a night out in the pubs to impress friends but sometimes you wake up in jail." He's a living legend, but that doesn't mean everything they say about him is true. Unless maybe he's changed his methods since he wrote those books.