When the players arrives at the bridge during Into The Pitt, Wernher offers the Lone Wanderer the option between a knife and a pistol. If the Lone Wanderer picks the knife, the Concealed switchblade is automatically added to the inventory in the Miscellaneous item section. The knife however cannot be used at this point. After the confiscation of your equipment, a switchblade in poor condition will show up in the weapon section.
The concealed switchblade appears only in The Pitt.