"DBO - UNDO Snap too Old" is a multiple-severity rule.
This alarm is invoked when the instance has generated a "Snapshot Too Old" error message. When update to data takes place, an image (snapshot) of the data block before the update took place is kept in the Undo segments, to provide a point-in-time picture of the data for read consistency.When running out of available space for new transactions, the database begins to overwrite expired undo operations; if the database space issue persists, the database may begin overwriting unexpired undo information. If current long-running queries require any of this overwritten undo information for a consistent image of the data, such queries will fail with a "Snapshot Too Old" error.
Each time data is pushed to DBO_Undo_Activity_Data topology the condition is checked and only if the it's true and there is no fired alarm, it will be fired. If the condition is false Foglight will clear the fired alarm.