jSteve,

Ideally you would check the area with a lab instrument to see exactly
what you are dealing with (fire dept's may have something). At the very
least I would buy (you can always return it most places) one of the home
CO2 detectors with readout to verify if it is CO2 or not. I think you
can now get them for propane and maybe gas but am not sure.

Regards, Ron