M+L Heart

When you get to the part where there is a brute holding a marine by his neck and asks "tell me its location" he will say "kiss my ass" after killing all the enemies go to the bridge where the marine was, near that bridge (off towards the path you came from) is a tree, jump in between the trees branches and if you look around you should find a carving that says "m+l (and a date)" surrounded in carved heart.