After the sneaking section when you reach the cabin with a missing lever, find this item on the floor to the right.
"Reims, 14 June 1915
We are on the retreat. Our position is untenable. Don't worry, I'm still in good health. We are going to leave Reims to the enemy. Reims is a beautiful city. I hope all is well at home and you are taking good care of our parents. I hope the prisoners are working properly for you.
See you soon,