We had an unused smoke detector that had been placed out of sight in a cabinet. It took us quite a while to find that one.
More ideas: Our refrigerator beeps when the filter needs changing. When the house alarm battery needed changing, there was beeping. Maybe it's the battery for your heat/ac temperature control panel.
I'm going to offer free advice to the Smoke and CO Association of Manufacturers (SCAM).
CHANGE YOUR AWFUL DESIGNS.
1 -- When the battery goes low......provide an LED that blinks for location. And then add a 10 cent storage capacitor with enough juice to power it til daylight (why do they always fail at night?)
2 -- Change the battery location from that woeful 'swinging door' design to a more human-friendly ergonomic. Replacing the battery on a step ladder, with a beeping alarm in your face, is a challenge.....especially at night (why do they always fail in the middle of the night??).
Rant Off.
Unplug/take battery out, whatever it takes to shut each smoke detector off. Do this one at a time for 24 hours each and see if you still hear the beep. Maybe one of your new detectors is bad. Also, look up the detectors manual and see what an infrequent beep means. Maybe you did something like not connect some wires correctly and one has come lose.
I hear the noise not like near a shower. 4 in a row small house.
Are you saying you are in a townhouse? Could it be coming from your neighbor's unit on a shared wall?