I think it's no coincidence that the library was considered a possible setting for murder in the game "Clue." Libraries can be downright creepy, sometimes.
Especially at 1 in the morning.
The library I work in is fairly small, for an academic library. Still, we have two floors worth of mostly bookshelves, with tables and carrels interspersed along the walls. Each night, before we close, we have to check to make sure everyone is out. I usually check the second and third floors, both because I work up on the second floor, while my coworkers work on the first floor, and because they usually have more last-minute things to do ("encouraging" students to leave the computer lab on time, handling last minute book check-outs, etc.).
Usually this is not a problem. I try to walk around upstairs a couple times during the evening, to get an idea of who's in the library and what's going on. One more walk-around at 12:30/12:45 is no big deal, until my imagination gets the better of me.
The rows of shelves and nooks and crannies inherent to a library make it very easy to imagine someone hiding in said nooks and crannies. From there, it doesn't take long to jump to the person hiding jumping out and scaring you, kidnapping you, or murdering you. The first semester I was here, I was still haunted from having gone to see "The Dark Knight" in theaters, and was sure I would one evening meet the Joker waiting behind a shelf. The other night, while gathering up books to place on shelving carts, I saw that one book was about photographing ghosts. I didn't go up to the third floor that night, relying on the fact that no one had been up there on my last walk-around.
Somewhat amusingly, I become much less nervous when I actually run into students on my last walk-around. I guess I figure that if an evil being is waiting to attack, I will now have back-up.
When my imagination really gets going, I can actually feel my heart start to beat faster. If I could bottle my flights of terror into workable stories, I could be the next Stephen King.
Luckily, no bad guys have actually been lurking in the shelves to date. Still, I'm happy that a side job ala Batgirl is not an actual requirement for being a librarian. I would never make it.