The best type of damp meter to use is a capacitance or radio wave type meter. This should be used at say 300 mm ccs over the floor and a contour map of the readings made The focal point of dampness can then be measured. Another thought is to use an infra red thermometer and see if the leaking water is hotter as there is better transfer to the concrete - you will have to do this just as it warms up and move smartly. As the pipe will be warmer nearer the start than the finish this is not a terribly good way of doing it but you may get lucky Chris