To find out who shared out the documents outside of your domain first you have to find the folder where files are located.
Then apply click on Show contents of this folder and its subfolders.
When the result is displayed, click on the Events tab on the top.
In the Events tab apply filter and search for Shared out = Yes
The result will show you all files which were “shared internally” at the start and then Shared outside your domain.
The email will show the person who shared out the document, the File owner will be the person who created the document, and the Event will be shared internally removed and shared externally added and the email of the external user shared with.
This can be set up as a scheduled report, so admin can view the people who shared docs externally, even if they are not the owners of the document.