> Capacitance may work although 6 inches may make it somewhat
> sensitive to other stuff.

If done the way a theremin does it, this will work easily, but larger
bodies at longer distances will also be sensed, and changes in
temperature and humidity will affect it too.  My theremin can see me
breathing quite easily.

I think PIR is probably the method of choice here, or doppler microwave.
They make car alarm sensors like that which can be set to low
sensitivity, operating around 1 GHz
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist