What could cause a false positive report from a line tester?
When wires that are not connected are in close proximity to live wires a voltage may be transferred through induction this is how transformers work. When voltage is traveling on a wire there is an electric field created the higher the load on the wire the stronger the field. Placing a piece of wire where it is affected by the field will allow it to pick up some of the voltage but there is really no potential or current. Many people call this voltage a phantom or ghost because as soon as the wire is connected to a load like a light the voltage is gone.