Posting this as the final puzzle is confusing the hell out of me.

The poster for this says "Left 5711". "Left" could possibly refer to the direction left or something being left open or left behind. As for the numbers, it could relate to rooms 5, 7, and 11 (or 5, 7 and 1) or floors 1, 5, and 7. So far I've tried:

• Leaving all the doors open except 5, 7 and 11 (and then 1 to replace 11)
  
• Only opening doors 5, 7, and 11
  
• Going to floor 5, room 7, and opening the door 11 times
  
• Going to floor 1, room 17 and opening the door 5 times
  
• Starting from the place where the skylight and poster are (room 5, floor 7) and going down 11 rooms

If anyone else has ideas on how to solve this puzzle, feel free to post.