Where I'm at:

Pulled down an ancient battery-operated detector in the 'old' part of
the living room (interconnecteds went up in 2003 during major
renovation) along with the interconnected in my bedroom. Left the
windows open that night (yes, I have a safe way to do it). No chirping.

Remained in this less-protected state until the weekend. Reinstalled
bedroom interconnected. CHIRP! Replaced bedroom interconnected.
CHIRP! Plus, I couldn't get the AC indicator light to illuminate. The
space for the wires (inside the junction box) is woefully cramped. And
the wires are triple strands twisted together; not the case with the
wiring for the unit I replaced in the living room when the saga began.
Three attempts to properly hook up the new bedroom unit failed. Removed
the back-up battery, drained the capacitor (i.e. held the test button
down until the residual charge died), put the non-functioning unit back
up (unconnected) for aesthetic purposes. Yeah, the door hangs open a
little without a battery, still looks better than a hole in the ceiling
with a junction box's guts hanging out. Installed the ancient
battery-operated unit in a more desirable part of the bedroom (further
from bathroom, ceiling fan, and fresh air sources than the
interconnected). After initial chirping that always happens when the
battery is replaced, the battery-operated calmed down and now protects
me in the bedroom WITHOUT CHIRPING THROUGH THE NIGHT!

Next question: Have I compromised my remaining two interconnecteds by
taking the bedroom out of the loop? What's with those triple-twisted
wires? The bedroom wasn't the 'primary' location for the interconnected
system with some sort of system-guiding functionality, was it? Because
if it's okay to do so, I might want to stay with a battery-operated unit
in the bedroom. I think an interconnected in the bedroom is gonna be
the source of recurring problems, given its proximity to open windows
and the ceiling fan.

R.