Apparently Paul found a note with the game, it reads as follows (this is taken directly from the annotation on the video, any misspellings or grammar mistakes are from Paul himself):
I WALKED DOWNSTAIRS AND WHEN I GOT TO THE BOTTOM, INSTEAD OF PROCEEDING, I TURNED THE RIGHT AND BECAME A SHADOW MONSTER MAN
Please go to my website on the sticker and also go to roneth's room and press start and press down down down down down right start
The first line appears to refer to the Flower Shack's stairwell; in episode 9, Paul pushes against the right side of the stairwell while wandering and clips to the ground next to the building, still darkened as if he was standing at the bottom of the stairs.
Little is known about the origin of this note. The way it is formatted indicates it may have been written by two different people (the capital letters in the top half, and the strange date in the middle of the note). The website on the sticker has not been revealed or mentioned by Paul. The stairs referenced in the top half of the note might be the same stairs in one of the hidden loading screen images:
This image can be seen around 4:20 in Petscop 2. You can clearly see a staircase, a door straight ahead, and another doorway on the right with light coming from it. We can assume this is where the mystery person in the note became the “shadow monster man”.
The second half of the note may have been written by a developer of the game, considering he knew the name of one of the Pets that according to Paul is impossible to catch. Also the fact that he knew the secret button combination indicates that he has a deep knowledge of the game.
The top half of the note may have been written by a child, due to the strange capital letters, the grammatical error “turned the right”, and the phrasing “shadow monster man”.
This probably references what happens near the start of Petscop 9, when Paul goes downstairs to Care's Room, but instead of going straight down, he turns right and Paul (Newmaker) turns black (like a "shadow monster man").