Well, what you are trying to do sounds a lot like steganography, which is a rather advanced version of the technique of labelling a tape of secret files with something inconspicuous such as "Gov promo 1989".

In other words, the trick is to masquerade the information as noise in a different file, which won't easily get noticed unless you know it's hidden in there.

http://en.wikipedia.org/wiki/Steganography

There are various tools for that, and you might want to have a look at this: https://superuser.com/questions/95877/steganography-software

Then there is this trick, mainly interesting because it relies on the often forgotten fact that headers are not to be assumed to always tell the truth: http://www.unixmen.com/steganography-in-ubuntu-hide-your-files-inside-an-image/

That said, I'm not sure if it's a great idea if your objective is a degree of actual security.

I would discourage even considering it unless you know very very well what you are doing.

Storing the information on a removable drive and physically locking it into a safe might be a step up − or a giant step down if, for example, your host machines somehow caches and indexes files on removable drivers.

All in all the best advice I feel like giving you, if you are aiming for security (as opposed to fun/experimentation) is: forget it.