Metal junction box shows 5V relative to ground when circuit powered. Why?
This sounds like a capacitive coupling issue -- the electric field radiated by the wires can and will couple to nearby metal objects, and a voltage-detector or high-impedance multimeter will pick it up.  If you can get a multimeter with a low-impedance (LoZ) voltage range, then that can be used to discriminate between capacitively coupled "phantom" voltages and voltage sources that have some "oomph" behind them.