This is an excerpt from J. Seal, M. Suttlar, A. Landrith and A. Kay, " Key Chain Locater," ECE 4522: Design II, Department of Electrical and Computer Engineering, Mississippi State University, Spring 2003:

The Key Chain Locator was designed to allow users to find their keys quickly and easily. The variable center frequency of the phase-locked loop allows each tone of the two tones to be distinct. This design feature makes the Key Chain Locator user specific. Future work will include designing the prototype to be smaller, more efficient, and to draw less current. The Key Chain Locator may even be integrated into existing keyless entry devices to allow multiple applications on one key ring.