What you have found is good as it is.
1. mirrors: mirrors are shaders, not textures. they use the textures/system/qer_mirror.tga file mostly as an in-editor image. The Yavin mirror uses textures/system/qer_portal.tga to be exact. And you can find the mirrors in the tests and system sets.
2. caulk_nonsolid and the others in the system set are shaders too. Their in-editor images are in the mapextras.pk3 in your base directory.