Thirsty Monk’s best bet would be to contact a local HVAC (Heating and Air Company). They would know the local codes and be able to hook him up with a detection system and auto vent. It might cost a few dollars, but it would be much better than a dead Thirsty Monk!
That being said, there are theses: http://www.instrumart.com/products/18180/ge-telaire-ventostat-series-co2-monitor that he might be able to hood up himself if he is a DIY’er.
Haven’t been in a McDonalds or had a soda in 20 years…
Combustible bottles travel in truck bed. After sticking my nose in fermentation freezer I know you can tell a high concentration of CO2 by the fact that your sinuses are burning off… But, an explosive release in a small compartment could be “interesting”. Not too worried about in the house. Same with the 1000 pounder. Low concentrations are not a problem, …(we drink the stuff remember) instantaneous release you don’t need a detector…But, leaks are $$, so for that reason I would consider it.