If something is where it's supposed to be, can you still call it a "discovery"? Suzanne Fischer, in the Atlantic, says "no."
It's an interesting discussion about cataloging archival material and the work that is, by necessity, still on the shoulders of researchers, spurred by the recent reporting of the "discovery" of a medical report filed by Charles Leale, the first doctor on the scene when Abraham Lincoln was fatally shot at the Ford Theater.
In the case of the recent press on the Leale report, the report had not yet been catalogued, cutting off discovery for ordinary researchers searching with finding aids and online catalogues. It's very possible, of course, with the volume of material that archives hold, for a particular professional to not know exactly what the repository holds. This is because archivists catalogue not at "item level," a description of every piece of paper, which would take millennia, but at "collection level," a description of the shape of the collection, who owned it, and what kinds of things it contains. With the volume of materials, some collections may be undescribed or even described wrongly.