Have a bad gaseous smell in home that comes and goes. Built home 12 years ago. Have oil and propane gas. Have had propane checked and did find 1 small leak but smell continues. Have had fire department out to test for gas, no detection.
Have central ac, basement, septic.
Have had plumber check drains. Does not smell like septic but a gas .
People walk in to home and say they smell gas.
It is usually contained to part of home that is above the room that holds the furnace but will drift up to second floor.
In basement I do have pipes in walls that are empty, put in if we needed to add more underground wires. They are covered.
Basement has flooded in the past but it is not a musty smell.

Are there any other issues/concerns in your home? Anything you can think of whether inside or outside? Some things could be related; I am only asking in order to get a bigger and clearer picture. Thanks

If you run your propane tank too low, you can get an odor. I had it happen once years ago. The propane people came and checked it out and said it was common and weren't too concerned about the odor under the my circumstances, but I did have them add propane and the odor went away. This is a possibility where you may not be using the propane and let it run out.