Well, this was just scanning the public/ folders, the files in which are accessible to anyone that has (or, in this case, can guess) the link.

Now, if it's something you absolutely must keep private, you shouldn't even store it in the private parts of your dropbox folder, but for things where it's not a huge deal if it ends up public, Dropbox is reasonably safe assuming you're not stupid about it (strong password, etc).