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As luck would have it, I can contribute on this topic.  I've managed
research on carbon monoxide and CO alarms since 1993 for a major U. S.
industry research organization.  The specific gravity of CO is slightly
less than air (I don't have my reference with me).  However,
distribution of CO in all but the most quiescent environments is
uniform.  This is not due to Brownian motion (although it plays a role)
as much as it is to normal convection, which is present in almost all
real enclosures.  In fact, sunlight on surfaces provides sufficient
energy to drive the required convection, even if
inflitration/exfiltration to an aircraft cabin is zero.  Several series
of experiments are available in the open literature describing CO
dispersion behavior, although the experiments are intended to
approximate dispersion in domestic spaces.  As a result, I would
recommend locating a CO alarm (1) where you can hear it, (2) where you
can see visual indicators such as a color change or digital display if
the alarm has such capability, and (3) where you can activate the reset
or diagnostics button, if present.  Finally, I would caution builders to
rely heavily on CO alarms for occupant safety.  Virtually all of the
sensor technologies currently in use have significant problems with
respect to interferences and degradation of performance over time, and
alarm quality (beyond the sensor itself) varies greatly among brands and
models.  CO from internal combustion engines is always associated with
other compounds the have strong smells (i.e., "exhaust fumes").  While
pure CO is "odorless and colorless," our concern is not pure CO as it
comes out of the gas bottle.  If you smell exhaust in your cabin, you
have a CO problem whether the alarm goes off or not and regardless of
how well you think your engine is tuned.  On the other hand, if your CO
alarm goes off at 20,000 feet, you cannot be sure that you have a CO
problem (or even CO) in your cabin.

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