Try using a drinking glass turned open end down on the surface of your questioning water pipe...you may hear flowing water, but if its simply a dribbling leak you won't hear it till it surfaces under you living room floor tiles.
I'm sure you didn't make a print of your water line routing did you? Neighbor had a leaking water line. It surfaced under his study floor. Simply cut the pipe where it entered and re-routed the water line outside around the abandoned water line.
You however may have a growing catastrophe that only Yingluck and her cronies can cure..