I wish those mains-powered ones would just shriek continually if the battery was low AND mains power failed; then you could kill the breaker and find it easily.
Yeah, I bet one somewhere has it, but most just flash-when-chirp which is kinda annoying:
This alarm has a low/missing battery monitor circuit which will cause the alarm to “chirp’” approximately every 30 - 40 seconds for a minimum of seven (7) days when the battery gets low. The chirp will be accompanied by a flash of the Safety Light. Replace both bat- teries when this condition occurs.
Some of the newer ones come with a built-in Lithium battery that supposedly is good for 10 years.
