Credits to Azukii from [Coup De Grace] for finding the treasure boxes.
After that, I decided to do a solo FSC run and purposely did not press the button. What I found out was that if you press the button, a blue box will appear. If you shoot that blue box, you'll get access to three treasure boxes:
If you don't press the button, no blue box will appear:
wait what, now I have to play fsc