"The files were inadvertently retained within other data files and this was not uncovered by the assessor," a statement from the chain said. (Our story from last week has been updated with the new information, along with a link to the earlier report of the breach.)
This is proving to be a frightening trend, with retailers believing they are compliant and much later on discovering various pockets of forbidden data scattered through their network.
In Forever 21's case, it only learned of the breach when the U.S. Secret Service called. Even after that heads up, the chain said it was unable to verify that it had been breached until the Secret Service walked executives through the incident and gave them more information (months later).
One of the problems in this case—and it could be argued it's a problem with PCI itself—is that it's up to the retailer's IT person to map out the networks for the assessor. If the IT manager isn't aware that someone from marketing had run a credit card experiment two years ago and that the files were never deleted (meaning that there might be live credit card data sitting in a marketing folder that IT would have no reason to ever look at), then IT can't tell that to the assessor.
In other words, the assessor has almost no chance of finding that data, making the certification much less meaningful.