There are uncharted mine shafts, some where homes used to be because people would mine under their basements. There are also sinkholes which you can see in pictures of the highway.

Can’t you map those by seeing where air is still flowing in? Release some dust and map its flow. Or better yet just cover the surface with foam.

Its not just mineshafts where it burns, it burns directly underground where it has never been mined before due to permeable rock.