I found a secret sign! Unfortunately the prize was something I already have a lot of
Viewing post in Lost in Firefly Forest comments
In the maze for the plunger, you need to go outside the maze to complete it, but that path goes further as well. The extra paths also only seem to be accessible from one direction (i.e. if you're pressing left while going up you won't enter the secret path, but you can if you're pressing left while going down)