
What rectangle? Oh do you mean the vent? Yup that's a vent, it is however too tall for you to reach, you keep thinking, but you can't find any way to reach it. Plus, you would need a screwdriver or something small and kinda like a screwdriver to open it anyway.
Turn off the lights and equip the Small Gameboy device and the glasses. Make a loud noise and wait for someone to come in. Use the Light from the Gameboy and use the glasses to strengthen it so it'll go bigger. When someone comes in, blind them then assassinate them and use their clothes as a disguise.
That would work if it was not for the fact that if you listen for 30 seconds you can clearly see there is no one guarding you. It's just you in here. Plus, you must understand that even if i say YOU i am actually saying that you are only a little voice in his head. He does not seem psycologically (Fuck this word) ready to kill someone.