Thisis a general circuit, which helps to search any device. It have a Transmitter and a Receiver. The receiver has LED light in it. When the transmitter is placed near by the receiver, the receiver lights glow. You can replace thes lights by any sound IC too. SO, when the transmitter comes near the receiver, teh Loud speaker connected to sound IC sounds.
The transmitter can be placed anywhere, which you feel as important. For example, if you had left your car in an parking area. Its very hard to find out your car in a rush. At that time, its just enough you had placed the transmitting circuit inside the car and left in the parking area. So, when you want to take it out from parking, its enough that you just walk with the receiver circuit. When your car nears, the receiver will start to glow. This circuit may also help if any vehicle is lost too.
So, this circuit also finds its application in security too.