Could someone explain how it is that someone discovers something that is already in a University archive? It seems people are discovering these kinds of things every few years.
Libraries do (or should do) collections management, whereby the librarians go through the records and find out which resources aren't being used and weed them. Ideally, if it isn't used, it's taking up space, and it's gone. This is balanced by acquisition, such that the net effect is a constant churn of what's there, so there shouldn't be much chance for anything to sit and molder where nobody knows where, let alone what, it is.
Archives don't do collections management of that sort. They do preservation, and ideally they never get rid of anything. Moldering (in a metaphorical, but hopefully not physical!) sense is very possible in an archive.
It's not exactly surprising that they might have forgotten or misplaced a few things.