A proper capacitive sensor consists of 2 plates, one connected to Gnd. That's how touch pads and screens are built, with many distinct sensors. A single plate is nothing but an antenna, catching whatever ambient noise.
How big is the piece of metal?You are right that the ground of the circuit as a whole affects sensitivity of the capsense, but it should still work if you use a good resistor value as well as capacitor value for stability.In case you haven't read the guide.http://playground.arduino.cc/Main/CapacitiveSensor?from=Main.CapSense
Also experiment with small capacitors (100 pF - .01 uF) to ground, on the sense pin. They improve stability of the sensor.
Every different behaviour, with a single plate, is based on different obscure and mostly unreproducible effects.
With a single plate you can get all sorts of effects.