Today I went into the kitchen to get some food. I saw some smoke that was staying at chest level. I checked if anything was cooking or burning but found nothing. The smoke didn't have a smell, and when I fanned the air it moved so I don't think I was just seeing things.
Are we talking opaque & coloured or translucent smoke? Different concentrations & compositions of gases vary in perception, heat also plays a part.
It's bad if the gas is noxious or flammable (particularly petroleum / propane). A thermographic camera might be cool to have around.
If we're talking about the other kind of smoke, you know whom to call.