Just because the exhaust was leaking into the home doesn't mean there was CO. No gas appliance should put off any CO unless it's not burning cleanly. The only exception really are gas ovens, those can and do put off way more CO than pretty much any other appliance, especially when dirty. So it's a good idea to keep the oven clean and use good ventilation while its in use.
Still, it's crucial to check all venting in all scenarios (like running the dryer and hood vent at the same time to see if it creates too much negative pressure inside and causes the flue to lose draft which would cause an exhaust spill out into the living space) because if it's leaking, the second it does start putting out lots of CO for whatever reason then it's life threatening.