Abstract: An elongated fluid detection device, together with a body of absorbent material, is located between the inner surface of a pipe and an impervious liner spaced from the inner surface thereof, the detection device being operatively connected to an external station. The impervious layer may be of a type normally used for relining defective pipes in situ, and the body of absorbent material, together with the fluid detecting element, preferably substantially filling the cylindrical space between the inner surface of the pipe and the impervious liner.