The type of proximity tester you are using is fine for quick checks on conductors and surfaces, but not always reliable. Since they will detect voltage at a short distance, it often gives false positives due to the fact that a hot conductor is close to some other part, such as your canopy. Often an AC voltage will induce a phantom voltage on adjacent metal conductors that your tester will detect.
The only certain way to tell, if in fact you have voltage on your canopy, is to use a Volt meter. Since voltage on a grounded part such as your canopy indicates a short to ground, it should trip your breaker. Regardless, use a meter and test the canopy to the actual ground conductor for any voltage. Also test hot to neutral and neutral to ground.
Another test you can try with your proximity tester is to drop the canopy and move it away from the hot wire. I bet the positive reading will disappear. Try this with the light on and off. If you still get a positive reading with the light off, you may in fact have a short to the casing of the fixture. If the positive is only when the light is on, then check to assure the neutral is not attached directly to the fixture case, as this will cause a positive reading.
Caution is always advised with working with electricity. If you continue to get positive readings or really don't understand the theory of what I have described, then get some help from an electrician to be safe. Good Luck.
The standard wiring pattern in USA (but not necessarily elsewhere in the world) for 110/120 AC is
black - hot
white - neutral
bare or green - ground
The hot wire brings the current to the outlet or fixture, often through a switch.
The neutral completes the circuit back to the panel.
The ground is a safety mechanism that, hopefully, will shunt the hot current to the ground (literally) rather than through a person if there is a short.
The terminals of a single pole switch (the only switch in a circuit and turning on/off one device or one set of devices) get the black wires, one on each terminal. Many switches also have a grounding terminal. This is not one of the switch terminals, but is usually a green screw on the metal strap of the switch frame, often on the opposite side from the terminal screws.
In general the white wires in a box should always be connected to each other, and never to or through a switch.
Similarly, the ground (bare) wires should be connected to each other and to the ground screw if the switch has one.
In your case, one set of wires (black, white, bare) are the source of the power, and one set (also black, white, bare) go to the fixture.
More complicated systems are possible with more colors, more switching choices and other configurations, but they don't apply in this case.
To work on this, make sure that the power is off (a non contact tester is best), before handling any wires.
Best Answer
It sounds like you hit a ground wire, was it still attached at the other end? If it was my house I would make access to get inside the wall to see, or buy one of those bore cameras and look in the wall that way. Please l`et me know if it was attached though.
Santa