In general, all of the gas detection devices on board have a built-in expiration date on their sensors. Those little "sniffer" chips might last a decade or even more, depending on ambient air quality, but unfortunately they do stop functioning.
Next, all sensitive electronics on board are vulnerable to salt air and vibration. The better-built ($) ones will resist Entropy a lot longer, but all will have their limits. Having installed and pondered the install instructions for both gas/propane detectors and CO2 detectors, my guess is that they are built to give an indication when the main sensing part has loss its mojo. Either with a light or the alarm itself.
First, are you sure that, for whatever reason, there is not a small amount of escaped CNG on board? Likely not, but trouble shooting has to start somewhere.
It's not unusual to find the expiration date on a tag on the back of the unit when it's unmounted. Those dates are often about 5 to 7 years from the manufacture date. (And that's not the date it was bought, after sitting on a shelf.)
You probably will have to start over with a new "sniffer" device, choosing from what's available for sale now. And, hoping to find something that covers the hole or surface where the previous one lived for decades.
