every time you change the shader in a single material and assign a texture, unity saves and remembers that properties/textures, even though you might not need it ever again. In general that's not bad and allows fast switches between shaders to test different looks.
Now i want to completely clean up my materials and eliminate all properties which are not used right now. I could find all mats in which the problem-texture is used, but even then I have to clean them up by hand via the debug view in the inspector -.-
Is there another or easier way?