I don't don't go along with induced voltages from other conductors. I think you are getting leakage resistance or capacitive effects across the switch. You have a much greater length of parallel conductors if your switch is on and your panel breaker is off so measured voltages shoud be higher. But they're not. If you have a Fluke 179, the input impedance is spec'd at >10Mohms. If you do the math, you come up with 105 volts dropping across >70Mohms to get the 15 volts that you are reading. If the resistance is infinate, <40pF of C would cause that much leakage at 60Hz. But just take the input impedance of your meter and multiply it by 7 in this case, to determine the amount of leakage resistance you're dealing with.
Joe
