They are the Lightmap meshes. When the Lightmaps are built all the static meshes are duplicated and their textures are additive to give the look that the mesh is brighter or darker in that area. When you delete or move a static mesh the lightmap mesh does not update so you're still seeing the baked lightmap mesh. There's no way to delete them in the editor except by rebuilding the lightmaps for the entire map. To build the lightmaps for a map open the console and type: buildlightmap levels/whatever.map
Depending on the map it could take a bit of time. Building lightmaps also correctly updates the games grid bounds which you will need to do if you ever create or move objects to an area outside of the existing grid bounds (which are invisible to you in the editor, but can be seen in game by typing showgridbounds in the console, but really don't worry about that unless you get an error message that says something about gridbounds).