Is your CO detector a combo CO and Propane detector? The propane detector is really a flammable vapour detector so any combustible hydrocarbons will set it off. Since you said that you poured antifreeze into the drains and if the detector is near the floor and anywhere near the drain it will set it off. The pink antifreeze that is safe for metal pipes will more likely set off the alarm more than the antifreeze for plastic pipes only as they contain a higher concentration of volatile hydrocarbons Once the volatiles dissipate then you should be good for the rest of the winter.
