I have already written a bunch of articles about smoke alarms (fire alarms). I would like to share my recent experience regarding causes of chirping smoke alarms.
Otis Fire installed a couple of fire alarms to a basement of a house, which serves as a daycare. We added the new smoke alarms to the existing hardwired smoke alarms. A couple of weeks after the installation, the customer called and told us that the smoke alarm (fire alarm) started chirping. The customer removed the smoke alarm, to check it out, a hand full of water dropped. They first thought there was leaky piping caused by the installation. After our investigation, we found out that water accumulation caused by the duct. The water flowed on to the fire alarm and caused the chirping issue.
Recently, I received a call from a customer, who replaced a chirping fire alarm with a new one. He said that the new fire alarm was also beeping. I went to the job site to investigate it. I waited by the smoke alarm until it beeps. I figured it out the beeping was not coming from it. It was coming from the old fire alarm that was replaced by the customer. He left the old fire alarm in the storage without removing the battery. Please keep in mind to remove the battery from an old unit because it will carry on chirping although it was removed from AC. Also, some alarms will continue beeping for a short period after all the power sources removed.