It may be different where you live, but in the US so far as I've heard, in all "mechanical" refrigerators intended for home use the refrigerant is a Freon® gas that is not "flammable." If your refrigerator is electric, and hums when it runs, that is most likely what you have.

The exception is for "convection cycle" refrigerators, used most commonly in recreational vehicles, running from "bottled gas," but with a very few using natural gas in homes. In RVs, the fuel is most likely propane here, but might be butane elsewhere. In these, the refrigerant is generally ammonia, which is, for all practical purposes, not flammable. Since refrigerators of this kind use a burning gas for power, a significant decrease in refrigerating efficiency could cause the burner to operate continuously, and more than normal CO would be produced.

Ammonia is also used as the refrigerant quite commonly in mechanical refrigeration units, but in most US areas is prohibited for "residential use" and is found only in "industrial strength" units, as for the freezer cases at supermarkets.

Any fluid that evaporates at normal ambient pressure and can be made liquid by compressing it can be used, at least theoretically, as a refrigerant. Early refrigerators (a half century or more ago) used a wide variety of refrigerant materials, but I haven't heard of a mechanical one using anything other than Freon (CF12 previously, now banned, and CF34, CF35 CF135, or CF125 etc in currently "legal" ones.). The only "gas" refrigerators in use here so far as I've heard are ammonia cycle ones.

If your refrigerator uses a gas for a fuel rather than having an electric motor/compressor, the constantly burning flame could have raised the CO level. The "flame" required in a home refrigerator usually is smaller than a gas burner on a cooking stove, and it is not commonly necessary to provide a separate "chimney" for venting.

"Pentane" is a hydrocarbon slightly heavier than propane or butane, and is a common component in "natural gas" - or in "liquified natural gas" (LNG) or "liquified petroleum gas" (LPG) that people use from a big tank in the back yard in lieu of a pipe to the supplier. Most people here call it "the propane tank" regardless of what gas is actually in it. Your reefer tech may have used the term "pentane" to refer generically to gas you burn, for the refrigerator or for cooking, heating, or hot water etc. Few here would use that term, but it may be fairly common usage elsewhere in the world.

The levels of CO detected by "color change" stick-ups usually are very low, so that you'll have plenty of time to notice an increase and take appropriate action. Electric/electronic devices with audible warnings can be set to trigger at somewhat higher (less sensitive) levels, since you get an immediate (and sometimes very insistent) warning and don't have to rely on someone happening by and "reading the color."

Since CO is "heavier than air" the sensors, of either kind, should be placed near floor level for maximum sensitivity/safety; but in areas where there is little air circulation too close to the floor may cause lots of seemingly "false alarms." Some CO is almost always around (if anyone is breathing), and in a "stagnant air" condition it will settle to the bottom. Manufacturer's recommendations on locations should be followed, for the device you're using. If the mfr doesn't give fairly specific instructions you should get a better kind of detector.

Some electronic CO detectors, like the Propane detectors required in US RVs, emit "test chirps" regularly, to assure you that they're working. I should be quite easy to tell the difference between these little "chirps" and the loud and insistent "warnings" if the device actually is triggered. In most cases if a good quality detector, properly installed, is giving excessive warnings, I'd be inclined to find out what's wrong with the environment rather than just blaming the detector. (A brief RTFM session to assure proper placement might also be in order?)

John