We had the '3 chirps' problem happen: 3 chirps, pause, 3 chirps, then nothing. It was fine for a couple of months, then it happened again. And again. As other folks in this thread indicated, it was the CO2 detector.
I looked up the First Alert manual to see what the chirp code meant. Just once/minute would be low battery; 3 chirps is the alarm indication, but would be continuous; 3 chirps, pause, 3 chirps is the test mode. Of course, no one was testing the CO2 alarm, so it was a puzzle.
I unscrewed the alarm to inspect it. It was clean, the LED was green, etc. Then, as I went to put it back, I noticed some movement: the black wire on the alarm was loose from its wire nut. If it was twisted just right, it would lose connection with the house power. So much for using an electrician!
If this is the problem for you: with power off, unscrew the wire nut, straighten out the alarm's wire, stick the alarm wire into the wire nut first, follow with the copper from the house, then twist to make it snug. Over-tightening can cause it to ride up and slip out. (don't forget to turn the power to the detectors on again)
I haven't had the problem again. As one writer indicated - it may have been related to a temperature change: after a few warm days, it got cool, and the metal may have contracted just enough to lose contact with the house power, and then recontact. The manual doesn't mention it, but it may be that the right pattern of random power connections gets interpreted by the alarm as a test. (like the old rotary phones: you could click the receiver to imitate the clicks for each number)