how to implement an electronic tether

I am trying to create an "electronic" tether between a user and a moving device. The concept I have is for the device to detect/measure the proximity of a particular individual who is 2 meters or closer. I imagine it as some combination of a proximity sensor and and RFID sensor. Has anyone successfully done something similar, or has anyone suggestions of how to approach such a challenge? Thanks in advance for any input.

There are a plethora of Bluetooth devices available doing exactly this. Do you want to re-invent the wheel? Google for wireless keyfinder.

Thanks - I'll look. I was clearly using the wrong search terms