Even naturally occurring swamp gas from sewers or marchlands is a common reason for getting false gas leak signals.
A common procedure to distinguish if gas comes from a leaking natural gas pipe or from any other source of methane, is sampling the gas in a bag, send it to an external laboratory and have the chemical compounds analyzed. However, this procedure is both time consuming and costly. Safety measures have to be taken and the leak treated as a true natural gas leak until the results are available, generally after one or two days.
The chemical compound the laboratory normally searches for is ethane. They focus on this since ethane and other heavier hydrocarbons are not present in near-surface microbial gas like swamp gas, but are normally a major component of thermogenic gases like natural gas. The ethane concentration typically varies between 0.5vol% to 15vol% in natural gas. A sample with presence of ethane would therefore indicate that the detected leak originates from a leaking natural gas pipe.