It would be great if a professional with specific knowlege of this subject
could answer this question.

I have read in many places that one should never leave unvented gas logs on
when they are unattended, but I cannot find out why. I would presume that
the reason(s) must be related to CO, O2 depletion, and/or fire.

Our unvented gas logs were installed in a modular home by the modular
company and strictly according to the manufacturers instructions.

The combustion products from these logs are disbursed throught an immediate
open area of about 15,300 cubic ft, plus up a stairway where the hot air
really flows to an additional 18,000 cubic feet. The total volume of air
in the living area of the home is about 40,500 cubic feet.

The logs are equiped with an O2 depletion sensor. A CO detector placed
near the logs has never measured any CO. The area around the fire place
does not become close to being too hot. The logs run on propane and
produce 21,000-34,000 BTU/hour. We ALWAYS run it on the 21,000 setting.
We have never detected any order

QUESTION: Is there REALLY any significant danger in running the logs
unattended? If so, is the reason one or more of CO, O2, fire, or something
else?

Many thanks.