• firstly, *nix based systems are... file based
  everything is in a file, somehow, somewhere; thus if someone gets access to the file system, they will eventually find it
  thus, keep your N9 safe, and data on it will be as safe
• 2ndly, hide stuff in plain sight.
  depending on what you want to hide you could consider encrypting it into a picture (aka Steganography). you probably have plenty of pics on your N9, thus...
  or put it in an encrypted ZIP (or TAR.GZ or... the more exotic, the better of course) with the -e or --encrypt option on the ZIP cmd line and put it e.g. in an software directory where you have tar.gz source packages (no need to do anything with them, obviously, just download a few); rename the file from file.zip to file.tar.gz et voilà...

if people are determined enough to find whatever you are hiding, they will find it, no matter how well you hide it. if nobody should see the information, destroy it.

and don't discount chance, e.g. someone seeing the picture and thinking «o, this looks nice, maybe i could zoom / crop it and use it as a background...» or «o what is this package doing? maybe i can try and build it and check it out....»