Tom,
I was never that sure. I carried my rods (bent coat hangers pieces through a couple of pen barrels) along a hospital corridor and when the rods crossed I was close to a covered manhole cover and a loo either side of the corridor. Too sensitive if anything!
Lots of what we called 'London' bonds in water (not sure if the same or a different term is in use these days) - that makes water such an atypical substance (freeze point should be far lower, minimum density at 4 degrees Celsius [so ice floats] and other properties).
When water is flowing, in particular, these bonds will be continually breaking and forming, causing this field effect, I presume. Certainly water carries a charge when flowing from a tap (the 'bending water trick' using a 'static electrically charged' rod.
So find where you known a drain or sewer is running and try it out.
Regards, RAB