It was officially stated during an online chat with the development team, called fight night. ( you can find the transcript for this by using MK5.org's search engine and searching for fight night)
Paulo garcia said there was one secret on all the platforms that had not been found. we DID know about blaze and mokap at this time so the final secret is NOT them.
there are many rumours, such as the whispering voice in the krypt, kenshis eyes, button combinations in various screens. but no one has found it yet.
whether it exists or not, it has become a regular mystery, and if you WERE to find it you would become a bit famous on here.
hope this helps. happy searching.

Thankyou i apreciate being up to speed! :D

I should've though about the title of the thread a bit more though!