metal detectors

We went to watch a movie last week with Charlotte. After the movie, we decided to go to the nearest coffee shop and went window shopping. When we went inside a boutique, there was a distinct sound coming from the walkthrough metal detectors. Charlotte was told by the security personnel to pass-through again the detectors and then it sounded again. Charlotte opened up her bag and pullout her dictionary and passed it through the detectors. The detector was able to read the barcode of her dictionary.