When the demo was released on CD in the UK, the source files included a second map inadvertantly left on by the developers. Ubisoft agreed to allow the level be posted for distribution and... well... here you go:
From what I have read, you will also need a second file in oder to play this new map. FarCryHQ has the file available called "Campaign.lua".
from FarCryHQ:
The Map-Files must be copied into the Demo-folder, the "Campaign.lua" must be overwritten in the Folder "Scripts / MenuScreens". Further instructions are included, don't forget to make a copy of the old files, because the mission "Fort" will be deactivated after installing the new map.