OK guys found some interesting things in this game you can say its spoilers...
















































so password for 1st keypad at safe room where you start is written on the wall 0921
password for 2nd keypad on board in training room is 1033815
there's and building you can reach by jumping through roof and there's hidden stuff there wont tell you what they are just look for it your self
password at keypad on board at that building is 103124 "you can find that password through cheats or on the hill at experiment room where developers tested stuff written there".
at the same building there are 3 doors one of them has keypad password is 0921 creepy stuff there.
and lastly found a way to enter that door in the sewers underground, to get inside you must type in the passwords on every keypad you find and you must find at least half or more collectable items on map like the green little doll that you shoot and give you points or manga pieces and erotic posters.
etc nice game little boring on waves, enemys do change here and there but no by a lot.
hope this will help for those who wanted to figure those little secrets.