Although it’s been ages since I posted, this is just a very quick update to post a link to the solution to the GCHQ 2013 Challenge, “Can You Find It”.
The puzzle is actually a set of 5 challenges, and is intended as a publicity and recruitment tool to generate interest in the agency. You can find a link to the puzzle here: https://canyoufindit.co.uk and a link to the full solution here: https://sites.google.com/site/gchqchallenge2013/
What looks most interesting is that there might even be a hidden/unannounced puzzle! See here for details: https://sites.google.com/site/gchqchallenge2013/is-there-a-hidden-stage