Level 3 - Cross Section 1 (Paper 3)
I think I missed it due to the fact its in exactly the same location as the boots and I was focused on them. With that in mind I figured it may be worth moving the note to somewhere more visible which can't be reached until the player has the steam boots. This would add extra game play to the areas and provide a good example of Steam Boot use.
The third version of the room layout shown above places the collectible note high up near where the player enter the level. This will be easy to see and requires the player to back track up the ladder once they have the boots.