The murder weapon has been discarded and you need to find it! The stash of the villain is also in the area, however this is harder to find and you'll need to solve 'Hitchcock Series: Secret Agent' to get at it.
At the GZ, look over the side of the river at the bank (the one you're standing on). You should see something. But how to get at it? I'm sure you'll work it out, bear the terrain rating in mind. Stay safe whilst retrieving, use common sense.
The terrain rating reflects both the walk to the GZ and the cache retrieval at the GZ. Wear wellies, you'll be crossing a stream!
The waypoints provided are the route I'd recommend - it's the least confusing way to go in this maze of paths and takes you past the most places of interest. You are, of course, welcome to make your own way there, but get ready to lose your way! Please note that you CANNOT access this area from the east (the Cwmtwrch Road). The routes are either private or too overgrown to navigate.
Note of interest: at the 'Cross the stream' waypoint, the bridge foundations you see are shown to still be a whole bridge in current OS maps.