No doubt the voltmeter is digital. When an energized conductor with AC power is near another conductor, a voltage is induced in one conductor by the other. But the amperage is so minute as to be impossible to measure with ordinary instruments. With a dial type voltmeter that depends on a current to operate the meter movement, the current is far too small to even make the needle move. Or to light a test light. I think you are reading "phantom" voltage.

It should be added that this is the case with the voltages encountered in house wiring. Voltages that are much higher, in the thousands of volts, are different animal.

The phantom voltage that busdriver is referring to is only only being measured because the "ground" wire that you are connecting/disconnecting from the switch is NOT electrically connected at the other end to the service ground. It SHOULD be, but it is not. It is only "floating" - and running with the hot wire of the lighting circuit causes it to show a phantom voltage when measured against something actually grounded.

You need to have this problem fixed ASAP.

OK,checked connection at service panel for this circuit. Ground wire twisted tight with all ground wires for 120 circuits & firmly attached to bus bar. Do not think the problem is there. Any one have any other ideas?

The ground could be detached at any of the points of connection between the outlet in question and the panel. You'll need to look in each junction box for the circuit until you find the problem.