AFAIK, RFID tags give out only information that they exist. You don't get any directional or distance information.
If you wanted to use RFID tags for locating something. You would need to have 2 RFID sensors quite far apart and use the strength of the signals to triangulate and find out where the tag is.
Any objects in between the tag and the sensor would degrade the signal and give false reading.
Also, small RFID sensors have short range (only a few cm).
I think its pretty unfeasible. But i have only done a bit of research.
Feel free to correct me
Oh and heres a link which you might find interesting - http://www.discovercircuits.com/H-Corner/smart-key.htm