It's been quite a while since I've posted, and I don't really feel up to getting into a hornets nest, but thought I'd throw in my .02...
Around these parts, it's difficult to find any type of underground line - even the utilities have trouble with lines. Case in point - while working on a water leak at my grandparents place a few years ago, we dug up a black iron gas main the gas company had no idea even existed. It was only a few feet deep, was leaking(anybody who has clay soil with a high water table knows how long that stuff lasts...) and was 30' (according to the gas company survey) from where it was supposed to have been. When they checked the spot where it was supposed to have been, there was a gas main there as well, so it was orphaned and forgotten about long ago, before my grandparents bought that place in the 70's.
When my uncle died, my parents bought a place just a short jump from my grandparents so they could take care of them. Yet, it was far enough away that my grandfather had no idea about any of the lines and such underground. When we went in to install a back yard drainage system, we called the utilities to mark their lines (knowing what little good that would do) and called my grandpa to get his take on things. He showed up with a few feet of #10 copper wire and a grin on his face.
He cut it in two equal lengths, bent handles on one end of each piece and took off. We have a barn at the end of the property (adjoining the road) and always assumed there was a wire coming from the house (the garage electric was ran from the house, so why risk it running from the garage) and sure enough, he found a line running from the house to the garage and the house to the barn. What he found next was unbelievable - a wire running from the garage to the barn as well.
I thought he was crazy, and so did his brother - the backhoe operator. Until, right under one of his flags, his brother hit one of the wire and cut it in two (without kicking a breaker) at the depth he said it would be. He was apparently able to tell if it still had electricity going though it as well (warning his brother it was hot before he cut into it with a pair of linemans pliers, ruining them and coming close to setting himself on fire with the arc). That day, I learned two lessons - Don't discount the old ways (People survived before the advent of digital electronics. Go figure, right?) and great uncles are flammable, or at least, their clothing tends to be.
Now, I'll admit, I have a college degree in electronic engineering and I have no idea how, why or if it works. But, if it works for you, go for it. If you've never tried it, grab a few copper rods, a case of beer and a shovel and give it a try - you are only out a few hours and some manual labor if it doesn't (or just a few hours if you have a backhoe). Who, realistically, has the ability, time or money to rent a Ground Penetrating Radar unit and operator for simple things such as finding a PVC water line. Around these parts, you either do the copper rod trick, do some old fashioned detective work or start digging "where you think it is". Usually you do a bit of all three, with fairly good results.
Perhaps it's all about perception - your subconscious telling you where something should be and it manifests in your hands. Perhaps it really works. I don't know and I don't think we'll know in my lifetime. But, it's not worth the cost of an aneurism debating it.