Oh, now I remember, I fucking forgor about the moving box. And the seggs ending does have an exclusive medal? I didn't receive it after the ending, or only the first ending you get that you receive a medal, kinda locking your save of the seggs medal if you get the normal ending first?Late to answer but anyway
Do you remember the hut after you get the forest key, but before the cave entrance? There's a room with a box, if you check that box from the side you can push it, and check the drawers to get the sack.
The reliable medal is an item obtained after achieving either the good or bad ending, only the seggs ending has an exclsusive medal.
And on the other comment about the theory of Ava, if I'm remembering right, Will doesn't have red eyes does he? I remember it being a trace from the mother but I could be wrong.