like some cloister that we know has tons of books but noone made a complete inventory list of them yet for whatever reasons.
or maybe some librariy that is so huge that there is a chance that in some chest is some books that noone knew they were there or existed ?
My favorite archive is the General Archive of the Indies in Seville, Spain. Anything that happened in the Americas from 1492 until the Republican Era is housed there. Since there are about 80-90 million pages of documents that take up about 5 miles of total shelf space, there is a great deal we have not found or seen. Most of it is inventoried by time and place, but there is a great deal that is generally random sets of documents filed away without any information pertaining to the contents. Every year or so there is a major find of some kind that tells us more about Latin American history or the culture of some forgotten indigenous group. I am holding out that one day we will have some set of letters or instructions on how to read Incan quipus (the knotted cords that we think we used for accounting). I remember that a few years ago, a friend of mine found a letter to the Governor of Florida in the 1600s written by an indigenous chief. What made it remarkable was that it was written in Spanish and then the indigenous language using the latin alphabet. For the first time, we could try to figure out how this language may have operated on a grammatical level and see where it might have gone in the historical record.
I know of an archives with undocumented texts - mine! And pretty much every archives. Archives as a general policy do not do "item level description" which is describing EVERY item in a record series. We usually do folder level, listing each folder on something called a "finding aid" which is like an inventory, or box level, giving a general description of everything in the box.
Usually this works rather well: say you have 3 boxes of meeting minutes from the local Key club or something, you say:
Key Club Meeting Minutes 1970-9
Box 1: Minutes 1970-3
Box 2: Minutes 1974-6
Box 3: Minutes 1977-9
And that's really all you need. This is sufficient for a researcher to effectively use the items in the record series. If you need the minutes from March 5, 1974, you can find them. But do we know every item in the archives? Heck no! And really, 99.9% of the time we don't need to, and we do not have the time or money (staff) to do an item-level inventory for everything.
So yeah, there's probably undocumented archival texts right in your city. And it ain't even spooky. :)