This issue was found while working with light mapping.

Steps to Reproduce
1. Create a new project, place down a terrain object, a light and a cube
2. In the search input area of the Hierarchy bar type t:Terrain to filter for terrain objects
3. Note that with the filter applied the terrain object is not listed.

Also, if you use the Scene Filter buttons in the Lightmapping window/tab to switch between filters you can also see that the terrain filter does not work.