What you're discribing is call phantom voltage; It's a result of capacitance between conductors. The high input impeadance (10 MOhms) of a DMM can cause you to get a reading on a conductor if it is in close proximity to another live conductor. Wires running along inside of a conduit will have high capacitance because the wire you are testing and live wires are running parallel to eachother over a long distance.
Shaun