In the rain over the weekend my neighbour noticed the ground bubbling 
between our two houses. The bubbles burned quite well too. Either we
were sitting on a Natural Gas well  or the PG&E gas line was leaking.
Unfortunately, the latter was true.

It was only noticeable because the ground was so wet that you could 
see the bubbles. 
It may be worth your while to take a look where your gas line runs 
during the next rain storm. I can only wonder how long the leak was 
there. Possibly years. At least the leak was before the meter.