Recently our local library has put in a very expensive security system. There are special gates you have to go through with your books and records or cassettes from the music library, which is on a separate floor. If by chance you've forgotten anything in your bag or pocket - or worse, If someone has intentionally hidden something - when you pass through the gate by the librarian a very sharp and deafening pipping noise begins. This hurts your ears and brings staff from all over the building to see who is trying to rob the library. It's not unlike going through the checkouts in a supermarket.
The other morning a mother and three children - two small girls and a boy - were going through the gates having their books checked. As the mother went through, the alarm went off and librarian waved to the frightened - looking mother to step back. She did so, and the noise stopped.