Thanks for playing! As for that secret, the answer lies in the same room as it. Can't spoil it. ;)
Indeed, I have since found it. If I have one criticism, it is that it is over too soon and both the Rocket Launcher and Plasma Rifle replacements barely get any use - even on Survival! On my end, I plan on tweaking ammo and enemy values to see if I can create more opportunities for those two weapons.
Thanks for making the map! I hope to see a sequel/continuation.