I recently removed a smoke detector to patch a hole and paint the
small area that needed painting. When I reinstalled the detector (120
VAC, battery backup) it started chirping. I opened the battery
compartment to find no battery. I have been in this house since March
and this detector has never made a sound before this.

I then checked the AC connections (advice from a friend) and found the
detector's wires needed more insulation stripped, which I did, and
reconnected. Still the chirping. I put a battery in and the chirping
stopped.

Question: Why would the detector suddenly start chirping (assuming the
cause was no battery) after I removed and replaced it when it didn't
do that before, when it had no battery?