This fascinates me, just did the secret route and it almost feels like there's a hidden story in a totally different Genre playing out in the background of the main VN. Kudos!
There's a basic walkthrough below but but you need to sit with Miko (let him stay quiet) at lunch, get a coat from Devon after losing your key, follow Klaus to the forest, embrace your inner monsterfucker (ie don't shove whoever accompanies you to the mysterious site and then pet the big spoopy monster.) After that basically do Bjorn's route (I personally did it while also keeping a distance from him but DO ask if you can room with him.) Then when you're sharing cake with him follow Klaus into the woods again.