Voltage in incomplete circuit
Volt meters are rated in mega ohms per volt. Most meters today are at least 1M ohm with many being 10M this so the meter won't load the circuit and provide an inaccurate reading. Since the impedance of the meter is so high it can detect voltage with almost no potential or what we call phantom voltage. There are low Z or low impedance meters out there that put enough of a load that phantom voltages won't be read, but in electronics trouble shooting low Z meters affect the circuit that's why most meters are high impedance.