I can see that, a trench can be an issue, as can certain situations under a house and such, I have run into a couple of situations where the conditions were just wrong when I was looking for a leak (while the gas was still on), but I could always stop the gas at it's source for my work.
We were the guys that cut holes in the street with jackhammers and backhoes, and twisted the gate valve off when the leak wasn't necessarily inside the building... and leaking gas follows the pipes underground back to homes and such... especially where you have houses with basements, as here where I live. A woman and service man died in a new house explosion caused by a leak in a yard line apparently from yard construction this year here.
My truck had snorkles on the front bumper to sniff for leaks outside. On the high pressure lines, you could look for dust being blown 200 feet into the air. On the IHP and LP lines, the computer will catch them, then they call in the emergency crew to find it and fix it.