RFC 822 requires at least one recipient field (To:, Cc:, or Bcc: line) in every message. If a message comes in with no recipients listed in the message then sendmail will adjust the header based on the NoRecipientAction option. One of the possible actions is to add an Apparently-To: header line for any recipients it is aware of. This is not put in as a standard recipient line to warn any recipients that the list is not complete.
The Apparently-To: header is non-standard and is deprecated.