If you are testing with a modern electronic meter it is not unusual to measure stray voltage currents due to the high impedance of the meter, at least that is what I suspect is happening, it is better to check with a resistance check, an insulation tester is even better but not always available.
If you use a voltmeter, use a resistor between 500 & 100 ohms, depending on how high the apparent voltage is, across the meter and see if the voltage collapses? If it remains the same then you have a problem, otherwise it indicates stray inductive or capacitive leakage.
This is just ONE of the reasons I prefer to build a system with all commons bonded to earth ground rather than the 'isolate everything' method.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.