On an unloaded power line - you can get voltage induced into that wire from a nearby source wire (e.g. solenoid wire). But in order to see this induced phantom voltage, you need to have a very high impedance input on your meter. A high impedance meter typically has a very high input resistance and adds very little loading to the voltage being measured. Very typical of a DMM type meter.

The old analog meters didn't have a problem reading unloaded voltages because they introduced their own loading into the circuit which eliminated the phantom voltages. DMM meters add very little load to the circuit being measured so they will sometimes see wildly high voltages on unloaded circuits.

First learned of this when training to be an electrician a thousand years ago. I used a DMM in an outlet to make sure it was dead -- it read 120VAC. Checked breaker - it was off. Remeasured - 120VAC. Guy training me laughed and moved an adjacent wire away by an inch or two. Lo and behold - voltage then went to zero.

Moral of the story - if you see a wild voltage on an unloaded power source, remeasure using an old analog meter to make sure it isn't a phantom voltage reading. Or throw a resistor between power and ground and remeasure.