The biggest cash heist in L.A. history didn’t go down at a bank. It occurred at a warehouse in a quiet corner of Sylmar. Early on March 31, thieves breached the roof of a Roxford Street facility maintained by security services company GardaWorld and stole as much as $30 million from the vault. The Easter morning heist drew sudden attention to an industry — cash logistics — that normally keeps a low profile and performs some of the roles traditionally done by banks, such as storing huge sums of money. GardaWorld and its competitors, among them Loomis and Brink’s, retrieve commercial bank customers’ cash in armored vehicles and bring it to facilities like the one in Sylmar, where it is organized and held on behalf of the financial institutions. Damage to a wall appeared to be repaired at GardaWorld’s Sylmar facility days after it was hit by thieves. (Myung J. Chun / Los Angeles Times) In theory, such warehouses should be easier to secure than banks, with far fewer locations to guard, entry tightly restricted …