It isn't uncommon for such items to lie undiscovered for so long. I recall back in 1970 walking through the woods on the Verdun battlefields. And as I looked up into the trees, I could see bits of leather equipment and even helmets hooked on the branches high above. It was later explained to me that as the trees grew, they dragged the relics of war upwards with them. Even now after all these years, much can be found lying on the surface. It is an offence in french law to remove anything though.