Extermination

Expert mode
Collect all 15 dog tags and finish the game. Save.
Begin another game with the yellow "Rear Of Compound" saved file
(00:00:00 time). The game will start with bunch of bonus ammo and
extra health items, but the enemies will be harder.

Secret mode
Finish the game by collecting all 15 dog tags and finding Roger's knife.
After the credits, save the game. Load back the yellow save file to play in
secret mode.