How do they know?

I can't seem to find a clock readout or a light sensor of any sort. How is it a smoke detector knows when it is two in the morning so it can give the low battery screech. I swear I have never had one go off in the day time. At least this time, the one that squawked wasn't one of the ones I need a ladder to reach. In their infinite wisdom, the house builders decided one of the smoke detectors would look great at the upper most part of the vaulted ceiling in the guest bedroom. That is a love joy to get to. I would move it, had they not cut a hole, put in an electrical box, and hardwired it in place. When the back up battery gets low, it honks, usually about 2AM. That one may be getting gutted and left to fill the hole. I can place one in the same room at the entry way where it would be within reach. I can reroute the wires in the attic. The one last night, a new battery didn't help. I hit the store and picked up a pack of four new smoke detectors that are white, and actually match the white ceilings, instead of the ivory ones the builder installed. It is the second detector to die, so I figure change them all and be done with it for a while. They actually thought to put the battery door on the front so you don't have to remove the whole detector to change the batteries. What a concept.