In the Develop module, (Lightroom 10.4) the message "Photo is missing" message appears under the histogram, yet the photo is not missing and editing features are active. Why does this message appear? It's grey but turns white when I mouse over. Library does not show the ! I'd expect if the photo was in fact "missing". Any ideas?
It is possible that the file is missing, but you have a smart preview of it (which would explain why the edit features are still active). You said there is no "!" mark on the grid thumbnail, but is there instead a white rectangle in the same top right-hand corner of the thumbnail? Under the histogram it should also indicate that you are using a smart preview, if that is indeed the case.