At Golden Rule Home Watch, we routinely check (and test) smoke and carbon monixide (CO) detectors in our client's homes. Frequently, the batteries are dead and/or the detector is faulty and no longer working.

See the picture of a USI Electric hard wired detector that expired in 2009 and started to beep continuously.  We have encountered other  detectors that did not make a sound when tested since they were no longer functional.