Nope, you dont have to do anything to make them appear. They can be any item in the scene, like anything else. Heck, I didnt even know there were secret items until I accidentally clicked on the black whale and then in another scene, the black octopus! When it brought me to the secret item book, I took note of what the shadow shapes looked like and tried to keep an eye out for them in other scenes. Not all scenes have secret items. There are 2 VERY hard ones to find and there is another thread in this forum that discusses how to find the safari hat, and the beetle. Perhaps thats one of those is the one youre searching for. Good luck!