This is a custom SFDC field, but it is mapped to the Marketo created SFDC field "mkto_si__MSIContactId__c (SFDC Lead)". The field contains no data and I can't find a reference to thiit in the help articles. Can this field be removed from SFDC and hidden in Marketo?
Did you ever get a response on this one? I've run a Field Trip on a client's SFDC instance to audit the custom fields and like you, find that this field hasn't been populated in any records. Did you delete it?