Why is the guitar amplifier acting like a radio when I touch the volume knob

This is a harder problem than I think you realize. The basic Loc8tor system works primarily by virtue of a directional antenna in the receiver, and it doesn't really give you the position of the tag directly, it just indicates what direction it is from the location of the receiver, along with a very rough estimate of how far away it might be.

If you want to create a mesh of automatic receivers, each of these receivers will need to do direction finding, either mechanically by physically spinning its antenna in a circle, or by using multiple antennas and electrically "spinning" the reception pattern. Neither method is going to be simple to implement or particularly low-power.

It is only by combining direction information from two or more receivers (plus knowing exactly where these receivers themselves are located) will you be able to derive an absolute position for the tag.

Your mic always "behaves like an antenna." The capacitance from your body only affects the resonant frequency of the "antenna." The resulting resonant frequency just happens to be in the AM broadcast band. This is why, when you touch the mic, everyone starts speaking French.

When you're not touching the mic, the "antenna" is tuned to best receive some other frequency than the Voice of Les Habs.

See this discussion on how RF gets into audio preamps.