The dang thing has been rarely doing the 3 chirp code a couple times during the day for the better part of a week now. Our cat is really frightened by the sound and I fairly quickly had a suspicion which one was throwing the chirps but I couldn’t catch it in the act to be sure, and it was otherwise ambiguous which one was at fault and there were no LED codes to help.
Finally confirmed it today. 10 years and 1 month past date of purchase. 10 year warranty. Figures.
It’s good practice to replace all the batteries at the same time regardless. If one is dying, they probably all are. Discharge rates will be different, so not all batteries will be fully dead. If you replace one fire detector, replace them all.
Safety equipment is best managed equally and a little bit of waste is acceptable. (Waste being: replacing a not dead battery in this case.) Consistent processes, no matter how simple, are important. Use the almost dead batteries for something else if you want, but not safety equipment.