The week 2 secret battle star is located in the Fork and Knife between Salty Springs and Fatal fields. Just go to the middle and search for the star in the small patch in the grass. If the star is not in the middle we suspect it to be in one of the other patches around the fork and knife. It could also be inside the knife and fork in the tunnels. Just walk around a bit if you cannot find it. You must unlock the week 2 loading screen in order to collect the secret star for week 2.
Or watch this small video for the exact location: