|Unless you're covertly stealing library materials or have unpaid fees, in which case just ignore and keep on walking.|
I don't know, maybe it's just me, but when I start to walk out of a place of business and an alarm sounds I generally stop and wait until someone comes over to 'clear' me of any alleged wrongdoing. That includes leaving the library. I guess enough patrons were just ignoring the loud, blaring noise they triggered upon leaving that it necessitated this sign. Note the "please" and "thank you" in it. Less accusatory and more polite. Miss Manners would be proud.