Among the many forensic faculties Sherlock Holmes possessed was one that allowed him to determine the typewriter a typed sheet originated from. Every typewriter left a unique fingerprint, and this is true not only in the realm of detective fiction: US courts have been using typewriter-based evidence for decades.
With the fading away of typewriters this forensic skill may have turned redundant, but printing technology has found a way to help the millennial variants of Holmes-like characters. From The Generalist:
Since the 1980s most colour printers and photocopiers add a set of secret near-invisible dots to every page they print. The dots uniquely identify the origin and timestamp of that printout.
Every printed page contained yellow dots that included the serial number of the printer / copier and a date stamp. It’s an anti-counterfeit trick, essentially. If someone prints out fake money, all that law enforcement needs to do is find the yellow dots with an ultraviolet lamp and decode them. Once decoded, they’ll know just where it came from (assuming, of course, that they can track the unique serial number) and when it was printed.
A cool technique perhaps, but it can be used as a surveillance tool too:
…these hidden marks were printed on everything… which is a problem if you are (for example) a whistle-blower in a totalitarian government and don’t want your leaked documents to connect back to you.