Fire alarm system 555 30th

The fire alarm system sounded today for no apparent reason.  I was not able to determine the source of the alarm.  It is in Zone 5 Fire apparatus bay and mechanical room.  I was able to reset it but it sounded again 15 minutes later.  A second thorough search was not able to determine the offending detector.  Repeated activation is not conducive to the function of the 911 center.  I elected to leave the system off line for now.   Perhaps someone with a different approach can sort it out and/or Jeff can show the dispatchers how to silence it themselves.  Otherwise a professional needs to be contacted if this continues to occur.  My next work day is Friday.  Thank you.