How can I trace water pipes and the water flow under the ground?
Here is what I would do to locate that plastic pipe under ground - I would disconnect it at your garage box, I would then insert a copper, aluminum wire of sufficient gauge to be able to feed into the pipe with out it curling up on you or you could run Electricians steel 'Fish Tape' .

At this point use a metal detector - you can buy them for about $100 - $200 U.S. 

Now using the metal detector you can plot where that Plastic Pipe is located ..Very easily..

Booyah!!..