Digital Watermarks on Printed Documents

"The question is how the government identified her so quickly, and the answer may be that she was inadvertently outed by the Intercept itself. That’s because the website posted an image of the leaked document containing an almost-invisible code applied by the printer that produced the document sent to the Intercept. The digital watermark identified the printer model and serial number, along with the time and date then document was printed out."

See LA Times