Smoke detector keeps chirping but there's no batteries in it? How?

Are there back up batteries in smoke detectors? I removed the batteries about 2 months ago and it was just sitting in my basement. Then all of a sudden the alarm chirps keep going off like crazy and I checked the batterie thing just to make sure and like I said, there were no batteries in it cause I took them out 2 months ago. Is it a capacitor that is holding the charge or what?