CrossRoads:
Stray voltage currents originating from what?
Usually from power transmission. Not an uncommon phenomenon.
Edit: Some general guidelines here: http://www.wisconsinpublicservice.com/business/farm_voltage_measuring.aspx
An Arduino should be able to take the place of the multimeter used to make measurements. Some additional input circuitry may well be required depending on the voltage level. Sounds like a good application for some data logging as well since a 24-48 hour measurement is preferred.