It started about 4 weeks ago, a squeak from somewhere in my basement. At first I thought it was a smoke detector with a low battery. So I took the battery out of my lower level detector, but the squeak continued.
After reinstalling the battery I started going through the basement. The squeak was only every 5 minutes or so, so there was nothing that I could follow to the source. Every time I heard the squeak I tried to zero in on where it was, but it seemed to move around.
There's an old alarm system from a previous owner, which we have never fully decommissioned. I figured it must have been this thing, beginning to act up. So I removed the backup battery (the squeak still occurred), I disconnected the power supply (the squeak still occurred); I bypassed every zone in the alarm system (the squeak still occurred) and I put it all back together. The squeak still occurred.
Examine the various modems and routers. Nothing.
Up into the ceiling...listening. Sometimes it felt as if I was getting closer but it was never as if I was next to the source.
Turn off the power supply to every basement room. The squeak still occurred.
What did it turn out to be? Someone had given me a smoke detector that I did not feel the need to mount, so I put it in a bucket that also held a cleaning brush and other basement randomness, on the floor behind a door. Eventually the installed battery began to fail, causing a chirp (my first instinct was right!) The sound went straight up from the bucket, so you couldn't really pinpoint where it was coming from.
Now I miss the damn thing.