lifehacker.com (a favorite of mine) posted up a free PDF book written by Charles Robinson titled, "Secret Hiding Places." Though it's a little outdated (1981), almost everything in the book still applies. I thought of you all first when I saw it, so take a look if you're so inclined. Link below:
Learn How to Hide Things in Plain Sight with the Secret Hiding Places Manual
Learn How to Hide Things in Plain Sight with the Secret Hiding Places Manual