The EURion pattern, with the circles, isn't exactly "microprinting" -- you can see it quite clearly without magnification on a lot of currencies. But there is also the later Digimarc system, and we don't know exactly how it works. Maybe somebody will reverse engineer the detection software.

Edit: as I noted on the list of printers page, we think that newer printers are also doing something that we can't see, possibly based on perturbing dithering algorithms so that the dithering is different from printer to printer in a distinctive way. So when we didn't see yellow dots from newer printers, that doesn't necessarily mean that they aren't printing tracking codes. The reasons for thinking that tracking codes became more pervasive in newer printer models rather than being phased out are suggestions in documents obtained via FOIA, and rumors from people who worked in the industry.