Home base alerts after smoke detector alarm

The last 2 times the smoke detector has alarmed, the home base will alarm again every 6 hours.  The SD does not siren again, only ther base station.  The panel says the SD alarmed 6 hours ago.  I entered the pin to turn off the base the first and second time.  I removed the device from the system and added it again but it alerted a third time.  I do not have the monitoring service.   Why does it do this?  How do I clear the alarm completely from ther base?