DLP Agent is unable to detect the secure file transfer while it is crossing the wire but it is able to detect the SFTP application accessing the file and trigger an incident (and block if configured).

In order for endpoint to monitor sftp activity the user will need to have application monitoring setup for the ftp client. The application monitoring settings will need to be configured to include application file access (AFA) so that endpoint can detect when the user selects a document to upload so it can be scanned accordingly.

Configure the sftp application here: