The work, “Girl With Balloon,” a 2006 spray paint on canvas, was the last lot of Sotheby’s “Frieze Week” evening contemporary art sale. After competition between two telephone bidders, it was hammered down by the auctioneer Oliver Barker for 1 million pounds, more than three times the estimate and a new auction high for a work solely by the artist, according to Sotheby’s. “Then we heard an alarm go off,” Morgan Long, the head of art investment at the London-based advisory firm Fine Art Group, who was sitting in the front row of the room, said in an interview on Saturday. “Everyone turned round, and the picture had slipped through its frame.”