I've bent coat hangers or other stiff wire & used them as Ludington said in #13, above to find underground pipe & conduits many times with great success- even deeply buried ones. Plastic, too. Rebar and large roots can trick them, however.
Bodies, too...
Once I had to find the bad cast iron underground DVW at my parents' house in order to break the floor for replacement. My mother asked what I was doing. I demonstrated by laying on the floor and having her walk over me, wires in hand. When the wires jumped across each other she screamed, dropped the wires and took off. I can imagine, being a good Italian mother, she ran upstairs and got her Rosary out! Maybe lit a candle or two.
If you prefer a locator, you can sometimes find them very inexpensively on eBay or Bonanza. After my truck got looted & my good locator got stolen for about the 10th time, I got a sacrifice Dynatel for less than $100.00 on eBay. You can clamp around the wire to the pump with some models (instead of de-energizing and tying to a wire.)
Paul