excellent engineering decision for smoke-alarm makers to set up the ‘my battery is low’ chirps such that they’re really far apart and seemingly intermittent in frequency. which one is chirping? who knows, make the homeowner work for it!
There's a sensible reason for it - need to have the chirping phase last a long time in case the owner isn't home for a few days. Worst case is the alarm running out of battery power entirely before the chirping gets noticed at all.