Smoke/Carbon monoxide detectors are fitted with a red "TEST" button, which emits a squawk when pressed,
to confirm that the battery is OK.  But you don't need to use it, because when the battery needs replacing,
the unit emits a squawk every two minutes or so anyway, right?    Wrong! Wrong! Wrong!
The 9V PP3 battery used can fail without warning to a near open-circuit condition, and the warning squawks will
then not be sounded. This is not a frequent occurrence but has happened to me twice.

So be safe, use the test button on your Smoke/Carbon monoxide detectors weekly!