The first time it happened, it was odd, but I assumed it was a prank. However, in 5 days, it's happened at least four times that I've heard of (Yellow, pink, and green are the colors so far; 641.5, 709, 823 and somewhere in the 100's are the dewey numbers that have been hit) and I'm beginning to wonder if someone doesn't have some severe OCD. Whatever reason they have for doing it, it's costing us a fair amount of staff time to fix it when we find it, and it's getting annoying. You think it'd be easy to notice someone walking back and forth among the stacks with books all of one color, but we haven't, yet. And if they're moving one book at a time to avoid getting caught with an armload of pink books, we'd likely notice the pacing/wandering to that one spot as someone that's either lost and needs help finding a book, or is possibly up to something or just pacing through the library because they're neurotic, or high.
The sad thing is that once they're caught, they'll be banned from the library for a year, even though rearranging books is a pretty stupid thing to lose library privledges for (as opposed to an offense that might be worth the risk, like stealing books or smoking crack in the bathroom).
cue William Tell Overture:)
I've named the villian the "Tome Arranger", since nothing helps an annoying situation like a bad pun.