I notice neighbors and ours too furnace vents that go out the back of the house. The "plume" of the vent seem to SOMEDAYS hang around the side of the house or creep up under the overhead soffitt even though the vent is 8 ft below it.
My ? is you can smell it if you walk by it but is this loaded with Co ??
If so I would think everyone should go UP thru the roof with the vents ???