I wouldn't be surprised if there are inkjet printers printing spy dots as well. I have an Epson Artisan 725 and I've noticed that it uses far more yellow ink than it ought to, given that the vast majority of my printing is black-and-white.<p>I'll have to do some testing to see for sure, though.
Earlier discussion of this and more sophisticated printer tracking codes:<p><a href="https://news.ycombinator.com/item?id=14501894" rel="nofollow">https://news.ycombinator.com/item?id=14501894</a>