Interesting. How close do you hold the lighter?
Right on the pipes, attention to joints and entry points thru the walls especially. When the flame grows in size, bingo. Gas leak. Of course do not do this if there is a strong gas odor. That’s self explanatory of course. But if you have that occasional Whiff and can’t nail it down, that’s how you find it. Then you know where to do the soapy water bubbles check. You would be amazed at how much gas is leaking from fittings.