Why do you think was the dagger left behind? Was it done on purpose? from the lesson ( journey to the centre of the Earth .)​

Are you referring to chapter 39? In the dirt they find a rust-covered knife and, knowing that it is not theirs, they conclude that some other man must have been there before them. On a slab of granite nearby this hunch is confirmed; the letters of Arne Saknussemm’s name are carved into the rock.