Due to the physical properties of natural gas, it has a distinct migration behavior. It is lighter than air, which means it will naturally tend to rise. Natural gas follows the path of least resistance and will travel upward through any available space, such as ducts, stairwells, cracks in the road or even soft ground. Leaking gas will flow out of open windows and doors naturally.
When underground or in enclosed spaces, natural gas can migrate, or move laterally. It will travel as far as it can under roadways and sidewalks, along natural gas pipes and other utility lines, or along a ceiling, until it finds a way up. This explains how natural gas can accumulate in buildings that are some distance away from the original leak site.
Even after the gas supply at an incident site has been shut off, be alert for gas accumulation and possible reignition nearby. And keep in mind that gas could possibly be leaking before a valve, so shutting off the valve may not mitigate the problem.
Specific safety information for transmission and distribution shutoffs is provided in The Natural Gas Delivery System Module in the Pipeline Valves and Closing a Meter Shutoff Valve topics.