No, not about storage caches when prepping for the Apocalypse. We’re talking Sealed Evil in a Can. Or Sealed Good. Or Lost Artifacts. Or any of a number of items and people that you want in your plot, but don’t want your general population to know about or encounter until the appropriate story moment.
So they have to be out of casual sight, but someplace your hero (or villain, or both) can get to. There’s a couple of good ways to pull this off.
First, and most handwavy, the hidden thing or person has always been there, you just have to meet the exact right conditions to see it, access it, or wake it out of its thousand-year nap. A lot of Masquerade worlds rely on Invisible to Muggles to pull this off. While that may be convenient for the plot, it may not easily jibe with either the original folklore (where usually anyone could see monsters) or with audience suspension of disbelief that anyone would make something reliant on a once-every-thousand-years conjunction or Ancient Bamboo Technology to keep things sealed up or open them when you need to.
(And then there’s the out-and-out ludicrousness of finding an artifact lost thousands of years ago depending on a glass bottle also locked away thousands of years ago that has to be placed in a statue… less than 1500 years old. Why yes, I’m still salty about Aquaman, why do you ask?)
On the more realistic end of the scale, maybe whatever it is, is just very far away. And most people don’t know about it because they don’t travel halfway across the “known universe”. This is easier to pull off in stories where there’s no reliable internet, but it can still work with one. Nobody knows every little nook and cranny of odd phenomena and odder people out there. How many people know about the Brown Mountain lights – and of those who do, how many would be able to draw a connection between them and Naga fireballs to figure out where a foreign monster might be hiding out?
In between, something hidden might not be that far away, but it’s difficult to find or get to unless you have very specific skills and info. In a shipwreck in the Caribbean. Part of a lost desert expedition (shades of The Mummy Returns). Or if you were trying to track down a specific note in an Italian soldier’s diary… when the last historical record of the man had him on mountain sentry duty on Mount Marmolada, 1916.
That would be a very hard guy to track down. There’s possibly 10,000 bodies up there no one ever found….
Which leaves open the possibility that (in fiction at least) not everyone on the fatality list was dead. Or that another, secret battle was concealed in the avalanches that day; the perfect plan to freeze a villain, or a hero, and provide your Sealed Evil/Good in a neat frozen package.
On that note – have Sabaton’s take on White Friday, that inspired this bit.