Person appears to have changed Segment, but there is no activity for that 'Change Segment' recorded in the Activity Log. Segmentation was not recently re-approved, so this would not account for the behavior we are seeing in other circumstances.
However, the record was recently converted from a Lead to a Contact in the CRM.
Upon Lead conversion to Contact in Salesforce, an "initial" segmentation process for that Contact is computed. During these "initial" segmentation computations, in a manner similar to when a person is initially created in Marketo, no activity is recorded in the record's Activity Log.