One Dog Story (EP)

Once you are in the Greenhouse section of the game, there is a door that will bring you back to the beginning section of the game (kinda). You will know it is the right room because Lilith will say something like "we are back where we started".

From here, head down and to the right. There will be a spiky maze of traps and obstacles. Just traverse the section and you will find the hidden weapon at the end.