Is there a dimmer in the circuit? I have seen dimmers bled thru to the light when the switch was off.
Actually dimmer, fan controls, and any switch with a pilot light will do this.
As to the OP, try putting a load on the two point you are taking a reading from, if the voltage is still there it is real, if it goes away it is a phantom voltage, putting a 1k ohm resistor across your voltage meter leads will also rule out phantom voltage.
Any wire that is not connected at either end but is run along with energized conductors will pick up a phantom voltage via capacitive coupling from the energized conductors, and high impedance meter such as most DVM's that have a 10 meg per volt input will read this voltage as they do not place a load on the voltage, by putting a 1k ohm resistor or other load across your meter leads, this voltage will disappear, solenoid type meters such as "wiggy's" or "Ideal Volt-Cons" place a load on the circuit being measured and will only report the real voltage, many digital meters are now being made with this feature using a selectable switch to place this load on the circuit being measured.