The key villain in the Sherlock episode His Last Vow the visuals constantly suggest that Magnussen has some form of electronic assistance to recall key facts that he can use to "leverage" influence over people in power.
We clearly see that Sherlock himself suspects that Magnussen has such a device but shows that it is not present, Google Glass-like, in his spectacles.
In the end Magnussen claims there are no files and there is no device. Is his claim credible? Were there no devices and no physical records of the blackmail material?