2017 Cascadia

Hello all, first time poster, long time listener.
I have a 2017 freightliner Cascadia 113 with a dd13, day cab.
I have a 3/8’s air hose coming out Going to nothing. On the drivers side as soon as you pop the hood right in there with a cluster of other air hoses... relay valve?
When I release the trailer brakes, air comes out of this mysterious hose. I put my finger over the hose and it sets the brakes.
Any idea why this hose is leaking or better question why is there a hose coming out of the wall leading to the brake buttons in the cab?
Thanks for any help

 

Ok I’ll see what it does. I can’t hear anything back at the back leaking anywhere near the air tank. Maybe the emergency relay valve on trailer is back feeding air?

 

Unhooked the blue line and no air coming back but still air coming out of that hose

 

Yea it’s driving me nuts. It’s not losing a lot of air but it’s annoying that I can’t figure it out