Conductors that are installed in close proximity to one another, and are capacitively coupled to each other, can cause this a.c. voltage reading. Such a reading could be 2 or 3 volts, or it may be as high as the voltage on the adjacent conductors. This is what is referred to as a “phantom” voltage.
Not my description. If you are checking that the circuit is safe to work on using a multimeter this is a problem you can run into. You are supposed to use a dedicated voltage tester.