Snopes:

Numerous brands of color laser printers leave coded metadata in barely perceptible yellow dots that can be used to trace a printed document to its source, a feature originally intended as a deterrent to counterfeiting currency with laser printers.

While a majority of laser printers are designed to produce this secret metadata, it is unclear exactly how many printing companies and models employ the technology.

I thought everyone knew about this. I remember hearing about it in the late 1990s as color printers were becoming more common in offices. We were told not to try and photocopy money – it wouldn’t work and we’d be tracked.