In my new house it had a loft that was converted into a bedroom by putting a wall up and a door. In the hallway there was a bookcase that was 16 inches deep and when I used a voltage detector I was getting a reading on the outside of the bookcase. When I would take the voltage detector and move it closer to the wall it started screaming really loud. I have never seen it where the voltage reading through the bookcase 16 inches out measured voltage. I open up the wall and there was a 14-2 wire going from a nearby outlet going up to the switch and another 14-2 going from the switch up to a fan in the room so I didn't notice anything out of the ordinary and it looks correct.

So what's the problem?