Nest protector chirp every 10-15 minutes for no reason

[Wertyui]

Help! 2 of my Nest protectors chirps once every 10-30 minutes. Sometimes it’s two short chirps. There’s no reason given on the message. Tech info says back up battery is ok, and expire date is 2029. How can I fix the chirp? Can I turn off all protectors? 

[justsam]

Are these wired or battery?  I assume since you referred to "back up battery" that they are wired.  Is there a dim green LED on perimeter of center button which indicates AC power is on?  Loss of AC power should show on nest app.  What environment are the units in?  Subject to any fumes, vapor, steam, etc?

[Wertyui]

My bad! I found an old school smoke alarm hidden nearby! The real culprit!