Daughter had a plumber in to do something to gas line or appliances.
I have no experience with gas but was there to let plumber in to work.

He had to shut down service to work and when done had to do a leak test before he could open service again and he detected a leak so had to chase it down and fix it before opening service again.

He found it in a fireplace that was seldom used in her basement.
No idea how long it had been leaking and apparently was not very big but I would think if house was closed for a long time buildup could have been a problem.

BTW no steel pipes involved in her place