DVD-rental vending machine maker RedBox today warned customers to be on the lookout for any unusual activity or physical changes to local RedBox kiosks, after the company discovered evidence that criminals had retrofitted at least three of the machines with devices to steal credit-card information. An example of a RedBox machine with an illegal credit card skimmer attached. The company said several RedBox machines had been fitted with “skimmers” — magnetic stripe reading and storage devices that can be installed over the top of existing card readers. RedBox said it found an illegal skimming device attached to one machine in Tempe, Ariz., and that it had discovered evidence of skimming at two other locations in Las Cruces, N.M. In a notice posted on its Web site, Redbox said is not aware of any fraudulent activity or transactions using its customers’ accounts, and that it is working to minimize the risk