This evidence is given by Brigida at the end of Crown Witness quest. After you check these orders for authenticity later they turn red instead of blue, thus you understand they are Falsified orders changing the convoy's route.

Journal entry Edit

From Baron Kimbolt to Horst Lubovitz
Honorable Sir!
I'm the last man who would tell others how to do their job, knowing even less about leading convoys than about carpentry - and I hasten to inform you that I deal with wooden items only in the privy. However, with the safety of Foltest's children in mind, I must warn you and suggest a change to your course.
The original route proved to be very dangerous and ridden with brigands, a fact that we, traveling ahead of you on the trail, experienced personally. Constable Natalis himself was lightly wounded by a treacherous arrow. Due to the importance of those in your care, and the responsibility resting upon your shoulders, I suggest you travel through Starling Valley, rest at the local tavern, and then follow the Old Trail, which Foltest successfully rid of robbers in his time.
Remember, should anything happen to those in your care, their blood shall be on your hands.