The term SIGN OF A NATURAL GAS LEAK can be summarized by the four letter word ODOR. Natural gas is an odorless, colorless gas that is used for heating and cooking in many homes. To make it detectable, an odorant is added to it so that people can detect a leak. The most common odorant is mercaptan, which has a strong sulfur-like smell. Therefore, the sign of a natural gas leak is the presence of an ODOR.