Maps that are used in materials are always reported as "referenced" (i.e. are highlighted in project panel), even if the material is not itself referenced

See the attached presentation for example. There is a main presentation and two potential subpresentations (they are not actually yet used as such), each of which shows a different textured rectangle. Regardless of which presentation is open, both images used in materials are shown highlighted in the project panel, even if the materials themselves are not used.