Gas leaks are hard to detect visually because natural gas is invisible and odorless. Suppliers add a substance called mercaptan to make gas detectable by smell, giving it a sulfur or rotten egg odor. If there's a gas leak, you might hear a hissing sound as gas escapes. An unexpected spike in your utility bill without an increase in usage is another indicator. Physical symptoms like headaches and nausea can also signal a gas leak.