Of course ours died at 3am while camping. Check the manual, ours beeped 3 quick beeps and only had a green light (red = CO) indicating per the manual that it had expired. We also travel with one of these....
https://www.aeromedix.com/Ultra-Low-Level-CO-Detector-CO-Experts-2014.html
Allows you to see EXACTLY how much CO is present. Any UL listed CO detector is forced to sit quietly until a certain level of CO is present over a period of time. The co experts model will alert you immediately.
Years ago, I was staying on a friends house boat at Lake Powell. I kept hearing this muted beeping noise.... zipped up in my duffel bag, in a closet, in a room 20 feet away, this detector was sensing the CO from an unvented burner on the stove. All the permanent (UL listed) detectors were silent. Call it peace of mind.