An air inversion could trap gas near the surface in an area, weather conditions would need to be just right with little to no wind for that to happen. Heavier than air gas could travel quite a ways and settle in a low lying area, once the wind picked up there would be very little evidence that it ever happened.
Yep. Common in certain areas of Africa. A large bubble of gas will work itself loose form the bottom, and release from the surface, then hover over a low lying area, killing anyone there. Only problem with your theory, though, is that there would be all sorts of dead wild animals all around them, too.
