Mohammed, you are ...

Mohammed, you are essentially correct, assuming that there are no significant temperature differences between the section inflow and outflow measurement points.  However, calibration and repeatability of the metering systems plays an important role, since meters calibrated with, say, +/- 1% calibration at different times can still have leaks up to 2% that are not detected. 

Unfortunately water main leaks due to corrosion and/or joint problems are fairly common, and frequently persist for years -- sometimes decades -- before they are noticed at the surface.  In arid areas such as we both live in, this constitutes a very significant water supply and distribution system maintenance problem.