The alarm may have been cleared automatically by the "Alarm Age-Out Time" functionality. The "Alarm Age-Out Time" functionality is defined in the "Alarm Management Subview" section of the "Modeling and Managing Your IT Infrastructure Administrator Guide" as follows:
Alarm Age-Out Time (hours)
Defines how long an alarm can exist in CA Spectrum. Once an alarm has existed for the number of hours specified by this attribute, it is a candidate for automatic removal. To disable this functionality, set this attribute to zero (0).
Every hour, CA Spectrum checks the status of all alarms in the landscape and uses this option to determine whether alarms are cleared. Therefore, an alarm is not removed at the precise moment when its existence time has exceeded the time-out. An alarm can be, at most, an hour "overdue."