Here's the fix, select only your skybox after you have it all set up around your map, make sure you use one brush as in one "cube" as a skybox with your map inside it, once you have your skybox selected/set up, go to the "selection" tab & go down to "CSG", select "CSG hollow", afterwards save your work, then re-build, compile, light, link & run it again, your invisible brush/texture problem should be solved when running the map.