Easter Eggs

In medieval ages messengers had tattoos under the scalp hair. Charles Dickens also described how women used to purl and to knit for hidden messages. Many software developers also insert messages or features in the code. The motivation may be to sign it or put some artistic touch on it – you will find a lot of websites out explaining the necessary keystrokes and web links.
I wonder if also other colleagues are hiding initials, words or messages in scientific papers? Unfortunately due to the online submission, publishers will now recognize faked references. What about using steganography to mark pictures or PDFs?