A pipe plug provides a water-tight seal and expedites locating of non-metallic pipes. The pipe plug includes a hollow body member adapted for being received by an associated piping member of a variety of inner dimensions, and for being engaged with it. The hollow body member includes a tapered portion with tapered threads on its exterior surface and a seal member made of a resilient material. The seal member envelopes the tapered portion of the body and is capable of moving along the exterior surface of the body in a direction generally orthogonal to the direction of the threads, providing thereupon a leak-safe seal. The body is provided with and engaged with a cap portion that includes a metal plate on its top surface, thus adapted for being located with a metal detector.