to get all 3 secret weapons, u gotta beat first 5 levels with silent assasin rating. also here's a tip. u can knock out guys using the anestetic, then drag unconsious guy and drag him over a sewer hole let them fall in, and when they wake, they cant clim back up due to an ai problem.

remeber to equip the anestetic on all missions, cause killing a single guard or firing more than 2 rounds per mission will not let u get silent asasin rating, and will not let u get the secret wepaons.

or u lazy guys can just type in invisible 1 and giveall codes in console to get all guns, and to not be seen.
to enable console, download latest patch at http://www.3dgamers.com and when ur in game press ~ key.