In 1345 he personally discovered a collection of Cicero’s letters not previously known to have existed, the collection Epistulae ad Atticum, in the Chapter Library (Biblioteca Capitolare) of Verona Cathedral
So basically a guy goes into a library, rummages for a while, and finds ~1400 years old text no one knew was there
Do we still have places that store texts (like libraries, but doesn’t have to strictly be a library) where we don’t have everything catalogued and we don’t know what might be inside?
While my answer was related to Africa, as that’s where I’m most familiar with a paucity of study and an excess of material, I’d bet even money that there are plenty of other non-European (and outside of the Americas) collections that have yet to be fully catalogued, especially in the Islamic World and India. I just couldn’t swear to it or name any regions in particular to look into.