Ah yes, you found the secret path. In my latest patch, I tried to make it a 'little' more obvious there was something there. You are on your way to winning! :D
That is the Spread Shot upgrade. It adds 2 fireballs shooting in different directions. That one is hidden near where you fought the first boss (about 2 rooms or so before you fight the boss). It is hiding in a 1x2 room and you will need to shoot a right-facing wall to open up a path to it.
