Art critic Alastair Sooke delves into the murky world of art theft. Despite the high stakes - and often daring - involved, many cases are shrouded in mystery and go unnoticed by the media. 　　Around 47,000 works of art are reported missing each year, yet it is only the heists involving the world's most valuable paintings that hit the headlines. But high-profile or not - once gone, the works are rarely recovered.