A museum, as you probably already know, doesn't display all the stuff it has on hand. It might like to, but it can't. There isn't nearly enough room. The museum only has so much space and they keep acquiring things that may or may not be display-worthy. Anything not on display goes into the archives, in back rooms and underground storage facilities that will often contain many times the amount of contents on display. That's where they maintain or restore it, that's where they research it, that's where they keep it when they're not taking a good look at it.
A lot of the time, an object in the archive has never really been looked at at all. The museum will take it after a comparatively cursory glance and store it, intending to get around to examining it more fully at some point down the line... and then it just sits there, sometimes for decades, without even having been cataloged. In Stealing Rembrandts, a book I reviewed back in June, at least some artwork is noted to have been stolen this way: if you're working in the archives, who's going to pursue you for stealing a piece of artwork when the museum never realized they had it in the first place?
On this page, from the Smithsonian American Art Museum, you will see a recording of some of their photograph archives. As an instructional aid, I direct you to the listing for the Walter Rosenblum Collection. The Smithsonian states that they acquired it in 1976. They also state that they are "in the process" of cataloging it, now, 38 years later. Another collection, the Library of Congress Copyright Deposit Collection, was acquired in 1987 and hasn't been cataloged at all.
And so now you have some idea of how a 6,500-year-old human skeleton, originally excavated from Iraq around 1930, turned up suddenly on Tuesday in the archives of the Penn Museum, run by the University of Pennsylvania. The museum was working to digitize the findings of an expedition by Penn and the British Museum that began in 1922 and ran for 12 years. It arrived at Penn in 1931, and originally went unrecorded by them because first, they didn't have anyone at the time who knew much about skeletons, and second, there was a little something called the Great Depression that was preoccupying a lot of minds at the time. So the skeleton sat there... and sat there... and sat there.
Until looters ransacked the Baghdad museum in 2003. At that point, for reasons that I will remind you what country this excavation happened in, Penn decided to digitize their Iraqi artifacts. Along the way- right now being 'along the way', because even when you start these processes it takes years- they saw a letter dated 1931, saying they were going to receive a couple skeletons. Skeletons are more of a British Museum thing, so even then, most of the Penn staff figured the letter was a mistake and they were actually in London (or maybe even Baghdad)... but eh, might as well go rummaging through storage just in case. And lo and behold, there was a crate that nobody had cataloged and that, because it wasn't cataloged, nobody in the anthropology department had any good reason to open up until then.
Luckily, it was very well-preserved, in wax no less, so it had held up. Penn has no intent of returning the skeleton, partially because they have a lot of research to do on it (skeletons from the 5500-4000 BCE era are vanishingly rare, especially in as complete a set as this one), and partially because again, there's a reason they decided to digitize the Iraqi collection and that reason has yet to subside over a decade later.
In case you're curious, they're naming it Noah.