It irks archivists when people talk about 'hidden gems' in the archives, because this lazy cliche tends to mean items which are in fact in the catalogue and may well have been publicised, but have not previously come to the attention of the individual who invokes this expression.
I am not entirely sure whether it is the same people who talk about hidden gems who think that in order to access the riches of an archival collection they need to speak to an archivist, preferably in person. That's actually what catalogues are for. An archivist can probably give you a general sense of what is in a collection, and possibly even more important, what isn't. But on the whole, the information on what we know about any given collection will be included in the catalogue description. I have processed many collections of archives during my career, and not all of them are entirely fresh in my memory by now, should you ring up and ask me about them.
On the whole, and doubtless with some exceptions, what archivists like to do is get the usable information about their holdings out there where people can look at it themselves. It should not be necessary, given a well-run archive, to need to know a specific name and possibly have a secret handshake ready in order to find out what they've got.
Or maybe this is just a version of 'do my research for me': the hope that talking to the archivist will obviate the need to actually look at the archives.