Rear Door Key: the key of the Early Access secret

The Rear Door Key is a physical item from Middle Town, in the First Sea, that only comes into play in the Early Access island secret. Once you have talked to Janus, the tester, it appears inside Robotmega's house, next to the Aura Editor, and it opens the locked door at the back of that same building. Behind it: a cutscene, then the tester role.

How to get it

Break the window above the broken cart, next to the Experienced Captain, and go inside: this is the Tester Dorm.
Wait for Janus, the tester, to come out of their room. The wait is longer than for the other secrets, and they only stay for 30 minutes at a time.
Talk to them, then go to Robotmega's house.
The key lies next to the Aura Editor: pick it up.

What it is for

Go around Robotmega's building: the rear door is locked, the key opens it. A cutscene plays, in which your character hacks into a computer to steal a "special system". Then go back to Janus: they give you the tester role and the secret is validated, with its 5 secret levels like every other one.

The Tester Dorm and Janus

The Tester Dorm is a secret area inside a Middle Town building, right above the Experienced Captain, and you can Flash Step inside once the window is broken. Janus, who lives there, wears two skins at once, which makes them look rather strange.

Good to know

The secret is called Early Access. As long as it is not done, the Secrets Master, in the central building, keeps the dialogue option "I heard you know a helpful secret about Middle Town".
The key has no other documented use.
Janus is the slowest NPC to show up of all the secrets: stay alert, or server hop.