What happened to the shop in one month?

Mr. Blue had a nice shop in the main street of a small town. He sold diamond necklaces, watches, clocks and other such things. All went well for some years, and then Mr. Blue’s shop was broken into at night twice in one month, and a lot of diamond necklaces was stolen each time. The police had still not succeeding in catching the thief three weeks later, so Mr. Blue decided that he would try to do something about it. He bought a camera, fixed it up in his shop so that it could photograph anyone who broke in at night, and put some very cheap diamond necklaces in front of it for the thief. A few nights later the thief came again, but he did not touch any of the cheap diamond necklaces that Mr. Blue had put out for him. He took the camera instead. It was worth $15,000.