Amusing note from the Nest Protect's user manual: Nest Protect has a limited life like any smoke/ CO alarm. Nest Protect lifetime is 7 years. You will need to replace Nest Protect after 7 years with a brand new Nest Protect. Check the date of replacement written on the back of Nest Protect.

It appears that, when it "expires" it chirps once a minute -- annoying the hell out of you until you replace it.