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The Signs, Beyond the Smell
Smell is the one everybody knows. PHMSA lists two more, and on a buried line they matter more, because gas escaping underground may never reach a nose.
Read this first
PHMSA lists sound and sight alongside smell: hissing or blowing, dirt being blown into the air, dead or dying vegetation over the line, and bubbling in standing water. A buried leak may never reach your nose at all.
If any of these is happening now, stop reading
Leave on foot, do not use a phone or a switch inside, and call 911 from somewhere else. Identifying the sign is not a reason to stay and investigate it.
By sound
PHMSA puts the range plainly: volume "can range from a quiet hissing to a loud roar depending on the size of the leak". A hiss near an appliance connection and a roar from the ground are the same category of event and get the same response.
By sight, which is how a buried leak shows itself
PHMSA lists discolored or abnormally dry soil and vegetation, continuous bubbling in wet or flooded areas, vaporous fogs, blowing dirt around a pipeline area, liquid pools, an oily sheen on water, and fire coming from the ground.
The one homeowners miss is dead vegetation. A patch of grass that will not grow, in a line, over ground where a line runs, is a sign in itself — and it can appear with no smell at all reaching the house.
Why the smell can be absent outdoors
Gas escaping below ground disperses into soil and open air, and the odorant can be filtered by the ground it passes through. That is why the visual signs exist as a category and why they are worth knowing on a property with a buried line.
What we do not publish
Any test you could do yourself. There are procedures involving soapy water and there are consumer detectors, and this site does not describe either, because the correct response to a suspected leak does not depend on confirming it first.