Gas smell in basement

Ok hear me out first…

The smell has been there for years. It is always strongest at the top of the steps when I have the door closed, like it rises and collects there. My basement is small, maybe the size of three parking spots. It has a gas powered water heater, boiler and dryer, a dehumidifier running almost constantly and an electric washer and fridge.

I have spent thousands having multiple different professionals out to look at it, they use gas detectors, spray that liquid that would bubble up all over the joints in the pipes, my one buddy even took a lighter to every spot there might be a leak…nothing. I’ve also bought a gas detector gizmo and nothing.

At my wits end but the smell is still there. Any ideas? Thanks!