Put something big on the end of the snake. A gasline fitting would work. Zero the metal detector in the place you will be holding it. You need someone to move the snake in and out about a meter while you hold the metal detector still. When you get a hit, mark it and move in a grid. Rezero, and repeat until you determine where the center of the strongest signal is. If it isnt too deep, you could use a powerful fishing magnet as well.