A printer can leave a pattern in the page
Color laser printers do more than place visible text and images on paper. Some also add a faint pattern of tiny yellow dots, repeated across the sheet. The marks are difficult to notice in ordinary viewing, but can be revealed by close inspection and read as a code.
The pattern is a machine identification code: information embedded in the printed page rather than stored in a separate file. Depending on the printer, it can identify the device by serial number and indicate when the page was printed.
How the dots carry information
The printer lays down the dots in carefully arranged positions. Their placement forms a pattern that a person or image-analysis system can interpret. Because the code is printed onto the paper, it can remain with a document even after the original computer file, printer, or packaging is gone.
That makes the marks different from a visible label. They do not need to be read by someone holding the page; they can be examined later, when investigators have a physical copy to inspect. The code identifies a printer, not necessarily the person who used it, so connecting a document to an individual requires other evidence.
A traceability feature with a tradeoff
The technology was designed to help track counterfeit documents, where knowing which printer produced a page could help trace its origin. The same capability also means that an ordinary-looking printout may contain identifying information its author did not intend to share.
In practice, the dots are a small but durable clue: a page can carry a record of the machine and printing time long after it has left the printer.
