The manufacturing date can be located on the bottom of the unit (2 digits) or stamped directly into the cylinder (4 digits).
This is usually caused by the absence of AC power to the smoke alarm.
Combination alarms save space in your home, and they mean you only need to test and change batteries in one device rather than two.
Any time a digital unit displays a number with or without an alert, assume it senses carbon monoxide. Leave the home and call 911.