It’s a no trespassing sign at the entrance to a tunnel on the Ruhr sieg nord route to the right of the tracks after a box with an f on it I watched a YouTube video for it player looks like he walked through a wall here’s the video Skip to 10:14
The collectables on RSN are bugged. There are too many of some jobs and not enough of others. Some are in broken locations too. This has been raised several times, but RSN Jobs haven’t hit the roadmap yet and I don’t know if they ever will.