Open the session related to the route if there is one.
Perform a Save-As and choose Create New Session and Route, or however it's worded. I'm not at my computer.
If there is no session related to the route, you can open the route up in Surveyor and do the save-as there instead.
This will give you a new version under your name. I recommend putting your initials at the end so you can tell the difference between the two. This will also preserve any consists and other settings for the route and session.
In your version, click on the tools icon at the top.
Click on Delete Missing Assets.
This will remove the errant unknown asset.