Hello again Tom. Some models have a low battery signal as well as an "End of Life" signal [I guess the alarm function wears out?].
Usually, the EOL signal is tied to ~60 months of use. I just replaced the apparently defective factory provided one in my 2008 2551 w/First Alert Carbon Monoxide detector w/signals noted above & everything seems to be fine now. I may have got a lemon on the factory one.
Off to McDowell Mt Park tomorrow.