How to track down a reference to asset?

I am trying to migrate assets from a sample project into a project which I want to use as a pseudo asset library. When I try to do the migrate, I often see things in the Asset Report that don’t even exist in the Content Browser like in the attached image. The report shows a folder “WaterPlane” and sub-folders and 1 asset will be included in the migrate but that asset doesn’t exist anywhere in the project (that I can tell) and the folder structure also doesn’t exist.

How can I track down and/or remove this erroneous asset from the migrate task list?