At the start of the game where the scientist rescues you, you make your way through the compound. You go past a CL suit testing a headcrabbed grunt, which (in true half life fashion) comes alive and kills the CL suit. Go past this, down the stairs and you will see Otis kill a headcrabbed scientist, and he says something along the lines of 'i've got another one!'. Let him open the door for you, and quickly shove a chair between the doors so they can't close (once they close they lock you out), and grab the Pipe Wrench. Go back to Otis, kill him and take his gun.