Yes, a probation office can find out about an arrest before indictment.  When someone is arrested, then the arresting agency will input information regarding the arrest into state and federal databases.  If, for some reason, the arresting agency forgets or neglects to input the state or fedederal data base,  (since this is still a manual process in some jurisdictions) then it is possible that the arrest information may not make it to the probation officer.  The probation officer can only access the data that is actually loaded into the data bases.