Any time a current is induced into a wire a magnetic field gets built up around the wire. If the magnetic field is in close proximty to another wire it can couple to that wire, thats how a transformer works. If you use a proximity tester (tick tracer) you can pick up these trace voltages. Sometimes ballasts and transformers can induce pretty high voltages on ungrounded metal conductors, enclosures, conduit etc. although the load carying capibilities is very small. You can read the voltage with a meter because of its high impedance. Just ground it and it disapates.
