When I was in middle school, I got hired to work in the local library. That you'd hire a middle schooler for that isn't the weird part of the story, although it's pretty weird. I grew up in a small community (2000 people or so, and it was a military base in the central Pacific if that matters), so there were only about ten library employees total, and we all worked just a few hours a week except for the head librarian, who worked full time (but wasn't there in the evening.) We had big meetings about every other week and you saw people during shift changes, so I knew everyone who worked there pretty well. (Plus I knew them all anyway, because like I said, small community.)

One of my jobs was to basically sift through all of the books and decide which ones to get rid of. (The library was going to be renovated, so we wanted to get rid of stuff before we boxed everything up.) It's a pretty subjective thing, but mostly I got rid of duplicates, books that hadn't been checked out for a long time, reference books that were way out of date, stuff that was in bad shape, and so on. Anyway, because I could do that part of the job at any time, I would sometimes work after the library closed. Anyway, the library had the sort of shelves where you can see through the shelf, above the books and below the top shelf. Sometimes when I was working nights after the library was closed, I was pretty sure I could see people moving a few shelves back, through a few shelves back. That's not really that weird, and is pretty easy to write off as an optical illusion. It wasn't more than momentarily creepy.

Then, at one of our meetings (I think they were every other week or so), one of the other employees mentioned seeing something similar. We agreed that it was kind of weird, but again, didn't really think much of it.

After that, though, all kinds of weird stuff started happening. When one of us employees would ride past the library at night after it was closed (it was on the only main road on the island), sometimes the lights would blink on and off really fast, slower than a strobe light or something like that, but still pretty quickly. If you were closing up and you sang or hummed, you could hear people singing or humming with you. That was creepy because it was easy to reproduce anytime you wanted to, unlike the other stuff which was more random.

What really makes me think of what happened as kind of different from other ghost stories is all the weird physical stuff that happened. Specifically, we all kept "getting stuff" while we were working normally. Like, you'd be shelving books, and you'd hold a spot open on the shelf with one hand while you checked the spine of a book in your other hand, and when you looked back at the hand that was holding the spot on the shelf, there'd be a spoon or something in that hand. It didn't feel like anything, and a lot of the time I didn't realize I was holding something until I walked away.

We got a few spoons (including a plastic spoon), some Scrabble tiles (H, U, U, C, H, F), and some assorted one-off things, but mostly we got keys. They were all basically normal modern keys, either the size for a normal door or a little smaller, like for a filing cabinet or padlock. Some were individual, and some were on rings with one or two others. We started keeping them in a big purple plastic bucket, and the bucket was almost two inches deep with keys by the end. They were mostly all different (I didn't do exhaustive pairwise comparisons of them, but they were mostly all different), and none of them opened up anything in the library or the main doors of the community services center, twice-a-week local paper, or bowling alley, all of which we shared a building with. (Yes, the library was above the bowling alley.)

What's weird about the whole thing is that it never seemed especially scary. I never felt afraid to be there, and as far as I know nobody else did either. The whole thing lasted maybe six months, although my memory of the timespan isn't particularly clear. I really wish now that I had thought to document the whole thing better. It was over fifteen years ago, and I moved away about ten years ago, so I can't even get pics of the key bucket or anything. Eventually we boxed everything up for the renovation, moved to another building for a few months, and when we moved back all the weird stuff stopped. We still had the bucket of keys and stuff, but AFAIK nothing that strange ever happened again. I almost wish that it had been a bit spookier, since maybe then we would have looked into it more deeply, instead of just joking about the library being haunted.

There's some possibility that the whole thing was a big prank, but it'd have had to have been a really elaborate prank on me in particular that everyone else was in on, and that doesn't make a whole lot of sense. (The singing and the figures behind the shelves would just be illusions, then.)