The essence of the implementation consisted in two devices: a receiver with a buzzer, which is fixed on the turtle, and a transmitter with a button, which works as a remote control. When the button is pressed, the transmitter sends pulses to the receiver and the pulses on its output contact open the transistor that powers the buzzer.
A friend of mine has a large turtle that escapes fairly regularly. He epoxied a tag with his phone number to the turtle's shell, and so far he has gotten the turtle back each time. He lives in a suburban area though, so there are enough people around. In a rural area the tag might not work so well.