Stray or ghost voltages occur from capacitive coupling between energized circuits and non energized, non connected adjacent wiring. Because of this coupling effect and the multimeter’s high impedance, it’s not always possible to determine if the circuit under test is energized or de-energized, and this creates confusion for the person performing the test.

Read more at How Stray Voltage Affects Multimeter Measurements by Fluke.

See also:

Fluke 117 Electrician’s Ideal Multimeter.

and

Fluke SV225 Stray Voltage Adapter Instruction Sheet.

Fluke 117 DMM from Amazon

Fluke SV225 Stray Voltage Adapter from Amazon