It must be a surface texture belonging to a material, as opposed to a background, otherwise the Library Loading Report would report a missing Photorendering Background.
And it's not a texture applied to a library part because that would either show, or simply not show without report or any appearance in the Attribute Manager, depending on whether it was saved as a 'non-editable binary 3D data' part, or an Editable GDL script.
Unfortunately you cannot use Find & Select to search for an element by texture, so you are S.O.L. there too.
Are you absolutely sure you have checked Attribute Manager thoroughly (ie. sorting by texture and chacking that column in alphabetical order)?Even if it is a hotlinked module it should show up in Attribute Manager.
It's weird - I have never been stumped by a missing texture. If you start a new project and use the latest settings, then reload your existing libraries, do you still get these missing textures? If so, zip it up and uplad it, so we can take a look - if you like.
Cheers,
Link.