Well, I've never made it go into alert mode. I'm talking about that really high pitched sound they emit just sitting there, while they're "scanning", or whatever they do.
THe reason they're at the library? Well, it's a noisy library for one, and second, they're placed away from the stacks and the only point of exit for the public, so if you want to leave with a book, you have to check it out and get it scanned.