The smell is a warning placed deliberately far below the danger
Natural gas is methane, and methane has no colour and no smell. What you are smelling is a sulphur compound added at the network entry point for exactly one reason: so that a leak announces itself while it is still small. That compound is detectable at parts per billion, which means your nose reports a leak at a concentration of roughly one per cent. Methane becomes explosive at five per cent. So the arithmetic is uncomfortable and useful in equal measure: when you can smell it, you are about a fifth of the way to the lower explosive limit. Not in danger yet — but not far, and the gap closes faster than people expect in a small kitchen with the window shut.
The range matters as much as the number. Below about 5 per cent there is not enough fuel to sustain a flame; above about 15 per cent there is not enough oxygen. In between, any ignition source works — and the ignition sources people forget are not flames. A light switch makes a small arc as the contacts part. So does a plug being pulled out, a doorbell, a fridge thermostat, the commutator of an extractor fan motor, and the static from a nylon jacket on a dry day. That is why the correct order is valve, then air, then look, and why "let me just turn the light on to see" is the sentence that appears in accident reports.
The other physical fact decides which window you open. Methane is lighter than air, so a leak rises and collects at ceiling level: open a high window, and do not go upstairs to investigate. LPG — propane, butane, the stuff in cylinders — is heavier than air, so it sinks and pools in the lowest point available: a floor-level vent, a drain, a cellar, the bottom of a boat. For LPG you open a low door and you do not go down into the cellar to look.
Three situations, three answers
A faint smell near the hob, and you can still work out where it comes from: shut the hob valve and the main valve, open a window, wait a few minutes, and then find it with soapy water — a brush of washing-up liquid and water on every joint, looking for a growing bubble. Hissing bubbles mean that joint; nothing means the leak is elsewhere or intermittent. A perished rubber hose is the most common culprit and the cheapest to replace, and a hose with a date stamp older than about five years should go regardless.
A strong smell, or a hiss you can hear: shut the valve only if it is on your way out, then stop touching things. No light, no fan, no phone call from inside, and — this surprises people — do not pull the main electricity switch either, because a breaker makes a bigger arc than a light switch. Get everyone out, leave the door open behind you, and phone the gas emergency line from outside, well away from the building.
The alarm is going and you cannot smell anything: treat it as real until you have proved otherwise, because a silent leak in a closed cupboard is exactly what the alarm is for. If a soapy check on every joint finds nothing and the smell is genuinely absent, suspect the sensor: domestic units have a working life of about five years, and the date is on the back. Past that they drift, and drifting means both false alarms and missed ones. Cooking steam, alcohol spray, hairspray and insect spray all trigger the cheaper semiconductor sensors, which is why the alarm goes off when you open the dishwasher.