Walter Noel and his family haven’t been seen yet this winter in Mustique, where the Fairfield Greenwich fund king has a big estate. Noel, who lost $7 billion of his clients’ money to Bernie Madoff, usually makes a big splash on the private island with his wife and four daughters and their husbands, who worked for Noel around the world. But sources say this year he rented the place to Bill Gates, who was spotted with his wife and friends at Firefly, one of Mustique’s fanciest inns. “The rumor is that Gates loves it so much, he plans to buy it,” said one source.