It's not a secret passage to the yellow button, you just need to deactivated both green buttons before you push the white button, keeping the gas in the room while also giving you access to the piss button. Then after you press the piss button it makes it safe to press the green button and let you get to the chest on the left. (Fun side note, I tested this out again to make sure I hadn't messed it up, and found that when you press the green button to get to the treasure it causes a visual bug, making the yellow wall look like it's down, but it still functions properly and blocks off the gas, lol. I'll have to fix that as well.)