Is there a way to trace water pipes through walls?
I was lucky and had limited access to the ceiling space that the pipe was running through by opening air ducts and pushing the vents up and out of the way. Using a webcam, a long usb cable and flashlight I managed to scope out the pipe in the space and trace it back to where it joined up to the main water line. Eventually I found the valve through pure chance. It was located inside of a wall stud and underneath a crossbeam rendering it all but invisible.