It began in late 2007 as a routine audit. Retail giant Wal-Mart noticed that some exit signs at the company's stores and warehouses had gone missing. As the audit spread across Wal-Mart's U.S. operations, the mystery thickened. Stores from Arkansas to Washington began reporting missing signs. They numbered in the hundreds at first, then the thousands. Last month Wal-Mart disclosed that about 15,800 of its exit signs – a stunning 20 per cent of its total inventory – are lost, missing, or otherwise unaccounted for at 4,500 facilities in the United States and Puerto Rico. Poor housekeeping, certainly, but what's...