Machine Identification Code
The grid of barely visible yellow dots that colour laser printers stamp across every page they produce, encoding serial number and timestamp.
The dots measure a fraction of a millimetre and repeat in a lattice across the whole sheet. Yellow on white is effectively invisible to the eye, but under blue light and magnification the pattern appears — on Xerox DocuColor machines a 15×8 grid encoding the printer's serial number, the date and the time of printing. No manufacturer documents the format, there is no standard, and there is no setting to turn it off.
Vendors added it in the 1990s under pressure from central banks and the US Secret Service, who wanted counterfeit banknotes traceable back to the machine that made them. The EFF published a decoding of several vendors' patterns in 2005. In 2017 the point landed rather differently: a scanned NSA report published by The Intercept still carried the dots, and they identified which printer — and therefore which employee — the document had come from.