I have picked up every single item outside of the buildings except for one last trophy. It's in Arkham West, and on the Riddler's Challenge Menu it's on the top row, right in the middle. It unlocks a reward. On the map it hovers way down at the bottom of the map. It is in between the medical facility and cave system, and much nearer to the cave system than the medical facility. I am not sure if I get to it on the actual Arkham West map, or do it from the Abandoned Tunnel that leads into Arkham East (it's right near the Arkham East gate, and it's not the one in the grate in the roof of the entrance).
Can't figure this one out either. I looked in the abandoned tunnel leading to the Batcave, in the tunnel leading from Arkham West to East, and near the door of Arkham West.
I think I know where to get to it though, I just can't figure out how to reach it. In Arkham East if you stand near the mansion and look directly west (away from the mansion), you can see there is a small cave on the far wall and in front of it is a stone ledge (looks like a broken bridge or something). Unfortunately, you can't use the Line Launcher to get to it because either every structure in the area is either too high, too low, or too far away. I'm almost positive that's where it is though. Just can't figure out how to get in there.
"If you come here, you'll find me...I promise..." ~Squall Leonhart "Tonight's the night we silence you, there's nowhere left to run." ~ADTR
Not sure if this is the right one but go towards the exit from Arkham West into the area behind the Intensive Treatment area. Before going through the door look up with Detective Vision and there is a pullable wall.