The duck with an equation on it also seems to have an uncompleted secret message.
This message could have means to find other secrets but this guide is to finding what the smudge says.

It's actually kinda simple, just like how you would unlock the paranormal achievement you need two ducks... The cyberpunk-like duck and the equation duck, put them close together and use the cyberpunk-like duck to scan the equation duck. Once the duck has been scanned you'll see a brief description of the duck you scanned of the side of your screen, it is there that you will find the ducks message in that description with the whole message and all.
;)