How do...

[Spectre]

... fire alarms with low battery know that it is night time?

 

[chookey]

lifeless cables turn into snakes and wrap themselves around every other single item sharing the same space?

 

[Spectre]

Maybe... Maybe... It's when they cool down the battery voltage or the sense circuit characteristic changes?

I swear it's 4am every time.

 

[snp-g]

Built-in timers that you can't change...

 

[miggy]

... fire alarms with low battery know that it is night time?

Chain smoking mice up all night partying probably. Coonts.