Regarding the expiration date. I've had MH's where the detector is original equipment and never replaced them due to expiration date and they functioned just fine. IMHO its cya from the company.
When they beep it almost always is a low voltage from the house battery.
And your house battery doesn't need to be dead either. Even a slight drop will set the alert beeper off. I think its a great alert for when my batteries/converter are getting tired. The last time mine started beeping it was found that the converter had a problem. :C